flexo
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What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 5:10 am

Ok, I found myself wondering what would happen if you were to lose the horizontal stabilizer in mid flight (i.e. seperated from the fuselage).
Obviously you would be in bad trouble, but would the nose go up or down? Intuitively I'd say id would go down before going out of control, but then again the horizontal stabilizer probably produces some lift so if it was lost it seems like the nose would go up instead.

Any educated guesses?

Also: Would there be any chance of keeping the aircraft under control after the loss of such an essential part?
 
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Starlionblue
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 5:20 am

Assuming you were in level flight and trimmed, the nose would go down. The stabilizer is pushing down on the tail to hold the nose up in level flight. Take away the downforce from the stabilizer and the aircraft would pivot around the wing. What the final attitude would be is anybody's guess.

Quoting Flexo (Thread starter):
Also: Would there be any chance of keeping the aircraft under control after the loss of such an essential part?

No.
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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:01 am

Quoting Flexo (Thread starter):
Ok, I found myself wondering what would happen if you were to lose the horizontal stabilizer in mid flight (i.e. seperated from the fuselage).
Obviously you would be in bad trouble, but would the nose go up or down?

Starlionblue has it right...the nose will go down. The horizontal stabilizer on all modern transports pushes down to maintain constant pitch attitude. The CG is ahead of the wing center of pressure, which tends to make the nose go down, and the wing itself has a moment that tends to drive it towards a lower angle-of-attack. The horizontal stabilizer has to balance out both these forces.

If you lose the stabilizer there's nothing to counteract these moments and the aircraft will lose control very quickly. This happened to a 707 in Luanda a couple of decades ago. It became a standard case study in the perils of fail-safe design.

Tom.
 
zanl188
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:05 am

Quoting Starlionblue (Reply 1):
Also: Would there be any chance of keeping the aircraft under control after the loss of such an essential part?

No.

 checkmark 

See the remarks at this website re: Aug 86 Aeromexico accident. This flight did not do well without its horizontal stab.

http://web.mit.edu/6.933/www/Fall2000/mode-s/collisions.html
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SlamClick
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:13 am

Quoting Flexo (Thread starter):
Would there be any chance of keeping the aircraft under control after the loss of such an essential part?

I cannot think of a single example of a successful outcome.

I have heard of surviving loss of vertical fin and rudder. I've survived loss of control of my rudder and a good friend lost control of ailerons and elevator but the sufaces and the weight & wetted area represented by those surfaces remained with us.

This is pretty much what it looks like.

I remember this one very well. I was in Vietnam when this happened and I don't think there was one aircrew member there who didn't see (and shudder at) this picture.

Sadly it was not only "friendly fire" but was a ceremonial round that didn't even "need" to be fired.
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EMBQA
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:14 am

Quoting Starlionblue (Reply 1):
The stabilizer is pushing down on the tail to hold the nose up in level flight.



Quoting Tdscanuck (Reply 2):
Starlionblue has it right...the nose will go down.

Not always the case.... Some horizontal stabilizers provide an upward lift....thus causing the nose to trim down. In that case, if you lost the stab the nose would pitch up. The Saab 340 is a good example.... it has an inverted airfoil for its horizontal stabilizer

[Edited 2007-07-30 23:16:33]
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Starlionblue
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:33 am

Quoting SlamClick (Reply 4):
I have heard of surviving loss of vertical fin and rudder. I've survived loss of control of my rudder and a good friend lost control of ailerons and elevator but the sufaces and the weight & wetted area represented by those surfaces remained with us.

This is actually an interesting point. The fin is important but not essential for survival like the stabilizer. The aircraft is (relatively) stable in the yaw axis since it is symmetrical and it is possible to retain control without the the fin and rudder.

As long as you do retain wing and stabilizer and no fin you can still control the aircraft with differential thrust, roll and so forth. Your results may vary.

Quoting EMBQA (Reply 5):
ot always the case.... Some horizontal stabilizers provide an upward lift....thus causing the nose to trim down. In that case, if you lost the stab the nose would pitch up. The Saab 340 is a good example.... it has an inverted airfoil for its horizontal stabilizer

Indeed. And there are relaxed stability designs. But in all cases the result will be the same.
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:54 am

Quoting Starlionblue (Reply 1):
Assuming you were in level flight and trimmed, the nose would go down.

This is precisely what happened to the lead P-51 in the recent Oshkosh accident. The left side horizontal stab was chewed up by a propeller, causing the airplane to nose into the ground. Fortunately for that pilot, he was already flaring for landing, and skidded to a stop with his tail in the air.


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EMBQA
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 7:04 am

Quoting Starlionblue (Reply 6):
But in all cases the result will be the same.

Correct.. but you can't hold what you said above to be 100% true every time. In some cases the nose will pitch up... not down as you said......but never the less, control be lost.
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 7:15 am

The IFR loss of control accident happens when the pilot loses situational awareness, usually entering the "graveyard spiral". This is a descending spiral with a bank angle exceeding 60 deg, and the first thing the pilot usually realizes is that he is descending rapidly, so he pulls on the yoke. This only tightens the spiral, does not halt the descent, and causes the pilot to pull harder. The result is that the tail fails from overload, the plane loses all control and the wings usually come off as well. It is invariably fatal. As to the initial question, the aft CG limit is always, I believe, still forward of the center of lift, so without the tail the nose will pitch down.

Quoting EMBQA (Reply 5):
Some horizontal stabilizers provide an upward lift....thus causing the nose to trim down. In that case, if you lost the stab the nose would pitch up. The Saab 340 is a good example.... it has an inverted airfoil for its horizontal stabilizer

I would dispute this. The fact that the airfoil is inverted indicates that it is producing down force, not upward force.
It of course would be possible to design a plane with a lifting tail; in fact this is what canard designs do. The disadvantage of canard designs is that the CG range is much narrower than with a conventional tail; I do not understand why. Anyone have any insight?
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777236ER
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 7:22 am

Quoting EMBQA (Reply 5):
Some horizontal stabilizers provide an upward lift....thus causing the nose to trim down. In that case, if you lost the stab the nose would pitch up. The Saab 340 is a good example.... it has an inverted airfoil for its horizontal stabilizer

I doubt the Saab 340's horizontal stabiliser provides a component of force upwards. To be dynamically stable, the centre of pressure must be behind the centre of mass, therefore to be statically stable, the horizontal stabilier should create a force vector downwards. Saab 340s aren't falling out of the sky every day, so the horizontal stabiliser is doing what all others do on commercial aircraft.

In fact, I seem to remember that the Saab 340 started off with a symmetric airfoil for the horizontal stabiliser. This caused dynamic stability problems with flap use, resulting in the inverted airfoil seen today.
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2H4
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 7:31 am

Quoting 777236ER (Reply 10):
I doubt the Saab 340's horizontal stabiliser provides a component of force upwards.

If I'm not mistaken, I believe the only horizontal stabilizers that provide upward force are those found on canards...


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jhooper
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 7:36 am

Quoting Flexo (Thread starter):
but then again the horizontal stabilizer probably produces some lift so

The horizontal stabilizer does produce lift, but in the opposite direction than the wings do. This helps in the longitudinal stability of the airplane. With the center of gravity being ahead of the lift, as the airplane speeds up in a dive, the downforce on the tail increases and this tends to bring the nose up. As the airspeed decreases in the resulting climb, the downforce on the tail decreases and allows the nose to fall. This creates oscillations about the lateral axis and allows one to trim the airplane to favor a certain airspeed. Without this stabilizing force, the airplane is uncontrollable.
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SlamClick
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 7:46 am

Quoting 2H4 (Reply 11):
If I'm not mistaken, I believe the only horizontal stabilizers that provide upward force are those found on canards...

As stated, not exactly correct. They would provide upward force any time the CG of the plane falls between the center of pressure of the wings and that of the horizontal stab. That is, in an aft CG condition in a conventional airplane and in a forward CG condition in a design with canards.

"forward" and "aft" are used in a relative sense here, not an absolute one.

Quoting Jhooper (Reply 12):
The horizontal stabilizer does produce lift, but in the opposite direction than the wings do.

Again, see the above.

Many planes could be loaded to a point where the tail is more or less neutrally loaded, or to some range either side of that. Most jet transports are designed to be loaded favoring aft CG stations and normal takeoff stab trim settings are in units of nose UP. But it could be otherwise.

In general aft CG gives less drag at cruise and therefore better economy. Forward CG gives better control.
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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 6:10 pm

Quoting SEPilot (Reply 9):
The disadvantage of canard designs is that the CG range is much narrower than with a conventional tail; I do not understand why. Anyone have any insight?

A canard design tends to put most of the fuel behind the CG, which causes a bigger CG swing than a conventional arrangement as you burn fuel. Since the total CG range is fixed by the control authority of the canard, the allowable CG range for everything else is much tighter.

Quoting SlamClick (Reply 13):
They would provide upward force any time the CG of the plane falls between the center of pressure of the wings and that of the horizontal stab.

If this happened, the airplane would flip on its back the first time it hit a gust. Operating in such a condition would be dynamically unstable and extremely dangerous.

Tom.
 
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SEPilot
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 8:13 pm

Quoting Tdscanuck (Reply 14):
A canard design tends to put most of the fuel behind the CG, which causes a bigger CG swing than a conventional arrangement as you burn fuel. Since the total CG range is fixed by the control authority of the canard, the allowable CG range for everything else is much tighter.

Thanks for the info, I did not realize this. I always wondered why the CG range for a canard was tighter.
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oly720man
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 9:53 pm

Quoting SlamClick (Reply 4):
I have heard of surviving loss of vertical fin and rudder.

http://www.talkingproud.us/HistoryB52NoTail.html

On January 10, 1964, Boeing civilian test pilot Chuck Fisher and his three man crew lost their tail, the tail of their B-52H Stratofortress that is, at about 14,000 ft. over northern New Mexico's Sangre de Christo mountains. Their mission was to shake, rattle and roll this big beast at high speed and low altitude to record sensor data on how such a profile affected the B-52's structure. They did their job. The vertical stabilizer blew off. Six hours later and after a lot of engineering on the ground and in the air, Fisher brought his B-52 home, with the coveted data.
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SlamClick
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RE: What If The Horizontal Stabilizer Was Lost?

Tue Jul 31, 2007 11:33 pm

Quoting Tdscanuck (Reply 14):
If this happened, the airplane would flip on its back the first time it hit a gust.

What ever are you talking about?

It can't work any other way.

The CG is the effective weight of the entire airplane. In unaccelerated flight this is always a DOWN force.
The center of lift in non-inverted flight is an UP force.
The horizontal stabilizer - elevator - trim assembly is the effort one puts against the lever to keep the whole thing level.

Now how could you balance a weight between the fulcrum and the force you are applying without exerting effort against gravity at that point? In other words UP.

If the weight was on the other side of the fulcrum from your force then it would take DOWNward effort to hold the weight up. Teetertotters would be really interesting to watch, were it otherwise.

By the way, planes don't simply flip over for a number of reasons, not the least of which is that the tailplanes act rather like the feathers on an arrow. The farther you try to displace them from "trail" the greater the airload pressing them back.
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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 3:49 am

Quoting SlamClick (Reply 17):
Quoting Tdscanuck (Reply 14):
If this happened, the airplane would flip on its back the first time it hit a gust.

What ever are you talking about?

Dynamic stability.

If the CG is behind the center of pressure of the wing you can be statically stable by having just the right amount of upward force on the horizontal stabilizer. However, if you run into a gust you will be unstable. As the gust hits the wing the angle of attack (and hence lift) will increase. This pitches the nose up, causing a further increase in angle of attack, causing a further increase in left, etc.

Dynamic stability requires that the CG be ahead of the center of pressure so that the disturbance results in a corrective moment, not one that increases the disturbance.

Quoting SlamClick (Reply 17):
The CG is the effective weight of the entire airplane. In unaccelerated flight this is always a DOWN force.
The center of lift in non-inverted flight is an UP force.
The horizontal stabilizer - elevator - trim assembly is the effort one puts against the lever to keep the whole thing level. Now how could you balance a weight between the fulcrum and the force you are applying without exerting effort against gravity at that point? In other words UP.

No argument from me there. The configuration you're describing is statically stable only though. It's dynamically unstable. That's why you only see such an arrangement in aircraft with augmented stability (i.e. fighters).

Tom.
 
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 4:20 am

In some trimmed flight conditions, the horizontal tail of a conventionally configured aircraft can be uploaded, due to the pitching moments created by the other components of the aircraft. However, it is usually downloaded.

Somewhere, I have a series of pictures of a B-17 where the horizontal tail gets hit and removed by the bomb of a B-17 above it. In the sequence, it noses over, like theory says it would.

Also, if the tail of a aircraft stalls, the same thing happens. On the first flight of the IsrAviation ST-50, when the flaps were put down, the tail stalled and the aircraft pitched down and went into an inverted spin. Luckily, the pilot (Norm Howell) thought quick and recovered by retracting the flaps.
 
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 5:20 am

Quoting Tdscanuck (Reply 18):
If the CG is behind the center of pressure of the wing you can be statically stable by having just the right amount of upward force on the horizontal stabilizer. However, if you run into a gust you will be unstable. As the gust hits the wing the angle of attack (and hence lift) will increase. This pitches the nose up, causing a further increase in angle of attack, causing a further increase in left, etc.

And this is why we invented moveable flight controls.

So you can adjust the amount of force acting upon the lever and compensate for the new force and bring the system back into equalibrium.

I do hope you realize there are a great many aircraft flying with CGs aft of the Center of Lift.
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 6:08 pm

Quoting Tdscanuck (Reply 18):
Dynamic stability requires that the CG be ahead of the center of pressure so that the disturbance results in a corrective moment, not one that increases the disturbance.

You're mixing up static and dynamic stability though. Static stability is when the aircraft will want to return to the trimmed AoA following an upset. Dynamic instability is when the aircraft will fly a phugoid of increasing amplitude at this AoA if left to its own devices.
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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 7:50 pm

Quoting FredT (Reply 21):
You're mixing up static and dynamic stability though. Static stability is when the aircraft will want to return to the trimmed AoA following an upset. Dynamic instability is when the aircraft will fly a phugoid of increasing amplitude at this AoA if left to its own devices.

You're right, my bad. Thank you for the clarification.

Quoting FredT (Reply 21):
I do hope you realize there are a great many aircraft flying with CGs aft of the Center of Lift.

Yes.

How many of them have augmented stability? For those that don't, how are they stable?

Tom.
 
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 10:37 pm

Quoting SlamClick (Reply 4):
I have heard of surviving loss of vertical fin and rudder.

How odd I was going to ask you guys about this only the other day... I was wondering if a modern plane could survive such a loss, and was therefore going to ask about the details of the importance of the fin.

When the AA airbus crashed after loss of the fin in New York,.. was that survivable, i.e was the plane controllable after the loss? Or did more that just the fin come away, the horizontal stabilizer too?

Regards,

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SEPilot
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 10:54 pm

Quoting Kieron747 (Reply 23):

When the AA airbus crashed after loss of the fin in New York,.. was that survivable, i.e was the plane controllable after the loss? Or did more that just the fin come away, the horizontal stabilizer too?

That's an interesting question; I have wondered about it as well. As far as I know, the initial event was the loss of the vertical stabilizer only. The fact that the plane was at relatively low altitude (the B-52 was IIRC at fairly high altitude) gave the crew far fewer alternatives as far as actions they could take; it also is important exactly what attitude the plane was at when the stabilizer departed. Due to the fact that the Airbus subsequently shed both engines after the fin and before it crashed leads me to believe that the fin departed when the plane was in a yawed position and proceeded to become totally unstable, whereas the B-52 was able to recover stability and therefore must have been in a relatively coordinated attitude when it lost its fin.
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Starlionblue
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RE: What If The Horizontal Stabilizer Was Lost?

Wed Aug 01, 2007 11:01 pm

Quoting Kieron747 (Reply 23):
How odd I was going to ask you guys about this only the other day... I was wondering if a modern plane could survive such a loss, and was therefore going to ask about the details of the importance of the fin.

The best modern example I can think of is the JAL 747 accident in which more than half the fin was blown off after a pressure bulkhead ruptured (incorrect repair). The aircraft limped on for a long time (more than an hour?) with very poor directional control before crashing in a mountainous area. Of the 524 souls on board, only 4 survived.

Quoting Kieron747 (Reply 23):

When the AA airbus crashed after loss of the fin in New York,.. was that survivable, i.e was the plane controllable after the loss? Or did more that just the fin come away, the horizontal stabilizer too?

Low altitude and speed. No time to recover. I believe the aircraft broke up in the air anyway, but I'm not sure.
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SEPilot
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 1:10 am

Quoting Starlionblue (Reply 25):

The best modern example I can think of is the JAL 747 accident in which more than half the fin was blown off after a pressure bulkhead ruptured (incorrect repair). The aircraft limped on for a long time (more than an hour?) with very poor directional control before crashing in a mountainous area. Of the 524 souls on board, only 4 survived.

I was thinking about that one as well, and am still puzzled as to why the B-52 was able to land successfully and this one was not. As you say, they did stay in the air for about an hour, so they did not lose control immediately as the Airbus did. Anyone have any thoughts?
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
SlamClick
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 1:18 am

Quoting Starlionblue (Reply 25):
JAL 747 accident

This event also severed lines on all the hydraulic systems. Whole different problem. The plane was almost 100% uncontrollable.

Quoting Starlionblue (Reply 25):
the 524 souls on board

Including Kyu Sakamoto of whom I can only say Shiawase wa kumo no ue ni. Shiawase wa sora no ue ni.

[Edited 2007-08-01 18:30:20]
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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 2:04 am

Quoting SEPilot (Reply 26):
I was thinking about that one as well, and am still puzzled as to why the B-52 was able to land successfully and this one was not. As you say, they did stay in the air for about an hour, so they did not lose control immediately as the Airbus did. Anyone have any thoughts?

Loss of hydraulics was a big factor in the JAL accident. The JAL flight didn't lose total control but they had an enormous phugoid-type oscillation. Unfortunately, after about 45 minutes one of the low points in their flight path coincided with a high point in the terrain. The plane skipped off the ridge and crumpled against the side of the next ridge.

Tom.
 
777236ER
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 3:03 am

Quoting MDorBust (Reply 20):

I do hope you realize there are a great many aircraft flying with CGs aft of the Center of Lift.

None of them are certified commercial aircraft.

Quoting MDorBust (Reply 20):
And this is why we invented moveable flight controls.

So you can adjust the amount of force acting upon the lever and compensate for the new force and bring the system back into equalibrium.

No, that's why we invented FBW. Control surfaces only control dynamically unstable aircraft when the instability is slight (Wright flying in pitch). FBW controls stability on modern dynamically unstable aircraft where the centre of mass is behind the centre of pressure.
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Starlionblue
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 5:04 am

Quoting SEPilot (Reply 26):
I was thinking about that one as well, and am still puzzled as to why the B-52 was able to land successfully and this one was not. As you say, they did stay in the air for about an hour, so they did not lose control immediately as the Airbus did. Anyone have any thoughts?

As mentioned.

Quoting SlamClick (Reply 27):
This event also severed lines on all the hydraulic systems. Whole different problem. The plane was almost 100% uncontrollable.



Quoting Tdscanuck (Reply 28):
Loss of hydraulics was a big factor in the JAL accident. The JAL flight didn't lose total control but they had an enormous phugoid-type oscillation. Unfortunately, after about 45 minutes one of the low points in their flight path coincided with a high point in the terrain. The plane skipped off the ridge and crumpled against the side of the next ridge.

There is a great docu about this. They show the flight path and it looks like the squiggles my daughter draws.

Quoting 777236ER (Reply 29):

No, that's why we invented FBW. Control surfaces only control dynamically unstable aircraft when the instability is slight (Wright flying in pitch). FBW controls stability on modern dynamically unstable aircraft where the centre of mass is behind the centre of pressure.

Indeed, "relaxed stability" aircraft are uncontrollable without computers since fleshbags don't have anywhere near the reaction speed necessary.
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777236ER
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 5:37 am

Quoting Starlionblue (Reply 30):
There is a great docu about this. They show the flight path and it looks like the squiggles my daughter draws



I think the chain of accidents caused by the loss of all hydraulics on large airliners is impressive. First we have JAL 123, a true disaster. From the lessons learnt from that, less lives were lost with UA 232 (I'm sure the deadheading UA instructor had tried throttles-only control in the DC-10 simulator because of JAL 123.) Then we have the DHL A330 in Iraq, which landed safely despite loss of all control. The crew of that aircraft directly quoted UA 232 as a reason they survived. Incidently, the DHL crew would be rightly celebrated as heros had they been carrying passengers.
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SlamClick
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 5:57 am

Quoting 777236ER (Reply 31):
lessons learnt

Unfortunately we also have the Turkish Airlines DC-10 near Paris and American 191 at Chicago which had way too much in common. Each initiating event led to the severing of more than one hydraulic system line, greatly decreasing the chances for a successful outcome. United 232 suffered a bit from this too, and only heroic efforts and no small amount of luck gave us a lot of survivors.

When hydraulic lines of more than one system are routed in close proximity a structural deformation can take them all out. But it's legal so it must be safe.
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777236ER
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 7:23 am

Quoting SlamClick (Reply 32):
When hydraulic lines of more than one system are routed in close proximity a structural deformation can take them all out. But it's legal so it must be safe.

Hopefully, we've seen the end of aircraft-wide hydraulic systems for new-build large aircraft with the advent of power by wire. The A380 has EHAs (Electro-Hydrostatic Actuators) and EBHAs (Electrical Backup Hydraulic Actuators), F-35 has some electrohydraulic actuators. In Airbus's case, they eliminated an entire aircraft-wide hydraulic system, and they'll improve on that with the A350.

Does anyone know if the 787 has introduced electrohydraulic actuators for Boeing?
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 7:29 am

Quoting 777236ER (Reply 33):
Hopefully, we've seen the end of aircraft-wide hydraulic systems for new-build large aircraft with the advent of power by wire. The A380 has EHAs (Electro-Hydrostatic Actuators) and EBHAs (Electrical Backup Hydraulic Actuators), F-35 has some electrohydraulic actuators. In Airbus's case, they eliminated an entire aircraft-wide hydraulic system, and they'll improve on that with the A350.

Don't they also have a bus instead of individual wiring links? That way the system will just reroute commands if a wire is severed.
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MDorBust
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 7:44 am

Quoting 777236ER (Reply 29):
None of them are certified commercial aircraft.

That's because there is no point to design an aft CG into a commercial aircaft.

Which doesn't change the point that the are a great many aft CG aircraft flying around.

Quoting 777236ER (Reply 29):
Control surfaces only control dynamically unstable aircraft when the instability is slight (Wright flying in pitch). FBW controls stability on modern dynamically unstable aircraft where the centre of mass is behind the centre of pressure.

And FBW does that how?

By manipulating moveable control surfaces.

FBW without moveable control surfaces would be one heck of a hoot to watch.

Quoting Starlionblue (Reply 30):
Indeed, "relaxed stability" aircraft are uncontrollable without computers since fleshbags don't have anywhere near the reaction speed necessary.

I thought the acrobatic SUs and YAKs had aft CGs.
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 7:47 am

Quoting Starlionblue (Reply 34):
Don't they also have a bus instead of individual wiring links? That way the system will just reroute commands if a wire is severed.

The EHAs and EBHAs run off three AC electrical buses on the A380. In the empennage of the A380 you end up with control surfaces powered by the green and yellow hydraulic systems, and AC1, AC2 and AC ESS. AC ESS can be powered in nearly every electrical condition, including from batteries.

Off the top of my head I don't think the EHAs can be powered by separate busses. I think a control surface powered by AC1 must be powered by that bus. The beauty is though that following a failure of an AC1 power source, the electrical system can be reconfigured to keep that bus going.

Because we're talking about power transfer through the busses - not simply low-power ethernet signal transmission, the EHAs are still susceptible to structural damage. However, if you're in a position where you've structurally lost two hydraulic systems and all your AC busses, you're probably a few thousand feet below the control surfaces and you shouldn't really care.
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 8:56 am

Quoting MDorBust (Reply 35):
I thought the acrobatic SUs and YAKs had aft CGs.

AFAIK no. If the CG was aft of the center of lift they would be dynamically unstable and need FBW for control.
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777236ER
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 9:38 am

Quoting MDorBust (Reply 35):
That's because there is no point to design an aft CG into a commercial aircaft.

Which doesn't change the point that the are a great many aft CG aircraft flying around.

Not that many.

Quoting MDorBust (Reply 35):

And FBW does that how?

By manipulating moveable control surfaces.

There's no need for the attitude. Control surfaces don't make dynamically unstable aircraft stable, FBW does.

Quoting MDorBust (Reply 35):
I thought the acrobatic SUs and YAKs had aft CGs.

Nope. Might have been close but nearly all aircraft are dynamically stable, F-16-onwards aren't.
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 11:00 am

Quoting 777236ER (Reply 38):
nearly all aircraft are dynamically stable, F-16-onwards aren't.

F-16 is only barely unstable. The list of dynamically unstable aircraft is actually quite short and does not include most modern front line fighters. I'm not sure about the one with question marks. And I probably missed one or two.
- F-16.
- SAAB Gripen
- Eurofighter Typhoon.
- Dassault Rafale?
- Sukhoi Su-47 Berkut.
- Mikoyan Project 1.44?
- F-117
- F-22
- F-35?
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SEPilot
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 11:34 am

Quoting 777236ER (Reply 33):
Hopefully, we've seen the end of aircraft-wide hydraulic systems for new-build large aircraft with the advent of power by wire. The A380 has EHAs (Electro-Hydrostatic Actuators) and EBHAs (Electrical Backup Hydraulic Actuators), F-35 has some electrohydraulic actuators. In Airbus's case, they eliminated an entire aircraft-wide hydraulic system, and they'll improve on that with the A350.

I can still conceive of a structural failure destroying all the buses leading to the tail but leaving the tail intact; the difference would be presumably the failure could be isolated so that ailerons, flaps, and gear could still be controlled. The problem with hydraulics was that when a line ruptured you lost the whole system; but I thought Boeing planes had check valves so that if a line near the tail ruptured the functions on the wing would still work. Is this true?
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 5:04 pm

Quoting Tdscanuck (Reply 22):
Quoting FredT (Reply 21):
I do hope you realize there are a great many aircraft flying with CGs aft of the Center of Lift.

Yes.

That wasn't me you quoted BTW.

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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 6:52 pm

Quoting 777236ER (Reply 33):
Does anyone know if the 787 has introduced electrohydraulic actuators for Boeing?

I believe they're used for some of the spoilers. I don't think they're used as extensively as they are on the A380 though.

Quoting Starlionblue (Reply 34):
Don't they also have a bus instead of individual wiring links? That way the system will just reroute commands if a wire is severed.

It's both. You can't run enough power to run the actuators over a data bus, but you can run the commands over the data bus. With redundant power and redundant buses, you greatly increase your chance that something is working.

Quoting SEPilot (Reply 40):
The problem with hydraulics was that when a line ruptured you lost the whole system; but I thought Boeing planes had check valves so that if a line near the tail ruptured the functions on the wing would still work. Is this true?

It's called a hydraulic fuse.
http://en.wikipedia.org/wiki/Hydraulic_fuse

Some of them are check-valve like, but they're usually more complicated than that. There are serveral types, but they all close the system down if something wierd happens downstream (a component starts taking way more fluid than it should or the pressure drop gets much higher than it should). These are supposed to isolate a leaking component and keep enough fluid in the system for the rest of it to keep working.

Quoting FredT (Reply 41):
That wasn't me you quoted BTW

Sorry about that...the original quote was from MDorBust.

Quoting SEPilot (Reply 40):
I can still conceive of a structural failure destroying all the buses leading to the tail but leaving the tail intact;

That would be a pretty magic bullet...what sort of failure are you thinking of? All the ones I can think of involved multiple independant failures.

Tom.
 
Kay
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 8:58 pm

Quoting Starlionblue (Reply 6):
The fin is important but not essential for survival like the stabilizer. The aircraft is (relatively) stable in the yaw axis since it is symmetrical and it is possible to retain control without the the fin and rudder.

As long as you do retain wing and stabilizer and no fin you can still control the aircraft with differential thrust, roll and so forth. Your results may vary.

What about that accident in NY in 2001 or 2002? It was an Airbus A300 (or A310)? there was turbulence from the preceding aircraft, and this one lost its vertical fin. This resulted in the aircraft oscillating its yaw left-right uncontrollably and just plunged to the ground.

After that accident, it was my impression that the vertical fin is the most important part after the wings or at least as important as the horizental stabilizer.

Kay
 
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 9:40 pm

Quoting Kay (Reply 43):
What about that accident in NY in 2001 or 2002? It was an Airbus A300 (or A310)? there was turbulence from the preceding aircraft, and this one lost its vertical fin. This resulted in the aircraft oscillating its yaw left-right uncontrollably and just plunged to the ground.

Low speed, low altitude, heavy turbulence. By all accounts after the fin broke other parts broke off too. Had this been at cruise with only the fin leaving the aircraft might have been recoverable.

[Edited 2007-08-02 14:41:16]
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tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 9:52 pm

Quoting Kay (Reply 43):
What about that accident in NY in 2001 or 2002? It was an Airbus A300 (or A310)? there was turbulence from the preceding aircraft, and this one lost its vertical fin. This resulted in the aircraft oscillating its yaw left-right uncontrollably and just plunged to the ground.

The fin separated because the FO made some enormous rudder inputs that took the fin attachment beyond it's static limit. So, when the fin left, there was a huge yawing moment on the airplane. Without the fin, there was no chance to correct that yaw and the plane lost control. Other bits separated shortly thereafter as they were subject to loads way beyond design.

Quoting Kay (Reply 43):
After that accident, it was my impression that the vertical fin is the most important part after the wings or at least as important as the horizontal stabilizer.

They're all important...if you could get away without any of them they would have been removed long ago in the interest of cost and weight saving. From a control point-of-view, the wing is probably the least important since it's the largest and therefore suffers the least proportional degradation when damaged. The horizontal stabilizer is essential for pitch stability...without it, you're dead. The fuselage gives you some yaw stability, especially at high speed but, without the vertical fin, you'll probably be unable to control any dynamic instabilities.

The vertical fin gets special treatment in some discussion because it is, by far, the most powerful flight control. You very rarely need to use the full control authority but it's there and when misused, as in the A300 accident over NYC, it can be a mess.

Tom.
 
Kay
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 10:34 pm

Interesting.

Is the scenario of an airliner coming in and landing without a vertical fin perceivable?


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3MilesToWRO
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 11:41 pm

I'll put it this way: do birds have vertical fin?
 
tdscanuck
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RE: What If The Horizontal Stabilizer Was Lost?

Thu Aug 02, 2007 11:57 pm

Quoting Kay (Reply 46):
Is the scenario of an airliner coming in and landing without a vertical fin perceivable?

I think it's perceivable. I'm not a pilot so I don't know how practical it is. The issue I can see is that you would have very limited yaw stability without the vertical fin. Provided you kept sideslip at or near zero it would be theoretically possible but I think in practice it would be very challenging. Especially given that loss of the fin probably implies some other strange happenings on the flight that aren't condusive to nice smooth level operation.

Tom.
 
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RE: What If The Horizontal Stabilizer Was Lost?

Fri Aug 03, 2007 12:13 am

Quoting 3MilesToWRO (Reply 47):
do birds have vertical fin?

Yes.

In a manner of speaking. Their tails can be rotated about their longitudinal axis, giving yaw control. An airplane's horizontal stab cannot do this.

Quoting Tdscanuck (Reply 48):
you would have very limited yaw stability without the vertical fin

That is true. Also the loss of several thousand pounds of structure at such a long arm would pose major pitch problems. Theoretically one might relocate passengers from the front of the plane to the rear of the plane until pitch stability returns. Also in theory, with wing mounted engines especially, one could use differential thrust for yaw control.

I'd rather somebody else attempt it. You can tell me all about it later. I'll buy the beer.
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