skychef747400
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Could An A380 Glide?

Sat Jun 30, 2012 8:44 pm

This is my first post after years of reading your posts, opinions and information. Unfortunalty I am a fustrated flyer who spends my days cooking but when I get on a plane I can assure you I aprechiate you guys at the front of the plane, the people looking after me in the back and those on the ground.
After watching documentaries on Gimli Glider and Air Transat 236 I have been wondering if a 380 was ever in that suituation god forbid where there was no engines would it still be able to glide. Please don't flame me too much  
 
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Polot
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RE: Could An A380 Glide?

Sat Jun 30, 2012 8:48 pm

Yes, all airplanes that can fly are capable of gliding.
 
BoeingGuy
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RE: Could An A380 Glide?

Sat Jun 30, 2012 8:55 pm

Quoting skychef747400 (Thread starter):
After watching documentaries on Gimli Glider and Air Transat 236 I have been wondering if a 380 was ever in that suituation god forbid where there was no engines would it still be able to glide.

It would be interesting to know what the A380's glide ratio is. The 787 glide ratio is supposedly just phenomenal, as was the A330's performance on Air Transat 236. I know about what the 787's is, but it's supposedly proprietary info.

The 777's glide ratio is more than 20:1 AFAIK. Older tail mounted airplanes like the 727 apparently have a significantly lower glide ratio.

Anybody know what the A380's glide ratio would be?
 
tdscanuck
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RE: Could An A380 Glide?

Sat Jun 30, 2012 8:55 pm

Quoting skychef747400 (Thread starter):
After watching documentaries on Gimli Glider and Air Transat 236 I have been wondering if a 380 was ever in that suituation god forbid where there was no engines would it still be able to glide.

Yes.

Taw intuition tends to tell our brains that, the heavier an object is, the worse it must glide. But that's not how the aerodynamics work. It doesn't matter how much it weighs, it matters how good the wing is (specifically, how high the lift to drag ratio is). Heavy airplanes have high weight but also high lift.

It all balances out such that, in the end, what matters is the L/D ratio. This ratio has been steadily improving for decades so the A380 probably glides as well as any large airliner out there except maybe a 787.

Tom.
 
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RE: Could An A380 Glide?

Sat Jun 30, 2012 9:09 pm

Quoting tdscanuck (Reply 3):
It all balances out such that, in the end, what matters is the L/D ratio. This ratio has been steadily improving for decades so the A380 probably glides as well as any large airliner out there except maybe a 787.

Tom.

Yeah, I figured we'd hear your knowledgeable input on this, Tom.   What's the 747-8 glide ratio? Do you know? Is it okay to post it, or is that also proprietary (in which case I wouldn't post it, nor expect anyone else to)?
 
JAAlbert
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RE: Could An A380 Glide?

Sat Jun 30, 2012 9:46 pm

Hey, where's the flame!?  
Quoting tdscanuck (Reply 3):
the A380 probably glides as well as any large airliner out there except maybe a 787.

A very interesting statement ...., are you saying that the 787's glide ratio is better than any other commercial passenger plane in the skies? What makes it so? How much better? What exactly does that mean? The engines go out and the plane can fly another 200 miles or what? (Please don't reveal the propriety good stuff, just a general discussion of what puts the 787 in a different category is all I'm asking).

Also, how will the 350 compare to the 787?

Quoting BoeingGuy (Reply 2):
he 777's glide ratio is more than 20:1 AFAIK

What does this mean? The plane drops 1 ft every 20 ft? 1000ft every 20,000ft? That seems a pretty impressive glide ratio if so, no?
 
peterjohns
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RE: Could An A380 Glide?

Sat Jun 30, 2012 10:33 pm

Quoting JAAlbert (Reply 5):
What does this mean? The plane drops 1 ft every 20 ft? 1000ft every 20,000ft? That seems a pretty impressive glide ratio if so, no?

Yes you are right- that s what it means. BUT how long does it take an airliner to clear 20.000ft? Keep in mind that is lateral distance. In cruise about 20-24 sec. Gives you a rate of descent of around 3000 ft/min. Still impressed!?

Of course in an all flame out situation one would adjust the speed to best glide, but it still gives you only about 15-20 min before touching down. I don´t even believe the 787 with it´s modern wing would have an advantage when it comes to L/D ratios, it´s simply not made for that.
 
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RE: Could An A380 Glide?

Sat Jun 30, 2012 10:33 pm

Quoting tdscanuck (Reply 3):
It all balances out such that, in the end, what matters is the L/D ratio.
Quoting JAAlbert (Reply 5):
A very interesting statement ...., are you saying that the 787's glide ratio is better than any other commercial passenger plane in the skies? What makes it so? How much better? What exactly does that mean?

As Tom has said the "capability" of an airplane to glide is a function of its Lift over Drag ratio. As you would expect from any modern commercial airplane the goal is to produce each unit of lift with the smallest amount of drag possible, therefore higher L/D ratios. Please note, that maximum L/D is different from cruise L/D, etc, etc, it is a function of multiple variables. Regardless, by simple inspection it would appear the 787 has a very high comparative L/D ratio and must therefore have an impressive glide ratio.

Quoting JAAlbert (Reply 5):

What does this mean? The plane drops 1 ft every 20 ft? 1000ft every 20,000ft? That seems a pretty impressive glide ratio if so, no?

Glide ratio is a unit less ratio. Therefore yes, if your airplane glide ratio is 20:1 you travel forward 20 distance units of 1 distance unit of loss of altitude.

[Edited 2012-06-30 15:34:56]
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rfields5421
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RE: Could An A380 Glide?

Sat Jun 30, 2012 10:47 pm

Quoting JAAlbert (Reply 5):
The plane drops 1 ft every 20 ft? 1000ft every 20,000ft? That seems a pretty impressive glide ratio if so, no?

Yes - 1 ft down for every 20 feet forward - assuming a constant speed.

The B767 and A330 were both at about 12:1

As an example just looking across the web - a hang glider runs about 15:1 - but a purposely designed sailplane runs 45:1 to 70:1.

Those are in still air at a constant speed. Obviously both are flown to see updrafts which extend the range considerably.

As far as more modern aircraft - glide ratio is a function of induced drag - thus smoother aircraft with better air-flows and lift glide better than older aircraft with less lift. Wingspan also has an impact upon the glide ratio.

Aircraft like the B787 (and I assume the A-380, B747-8 and A-350) have benefited from advances in technology to minimize the drag, make better use of their wingspan, etc.

Many of the things which make the aircraft more fuel efficient and longer range are the same things which would make the aircraft glide better if necessary.
 
tdscanuck
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RE: Could An A380 Glide?

Sat Jun 30, 2012 10:51 pm

Quoting BoeingGuy (Reply 4):
What's the 747-8 glide ratio? Do you know? Is it okay to post it, or is that also proprietary

As far as I know, it's proprietary. There's a very good public paper on the 747-8 development that includes a discussion of L/D changes on the 747-8 but it's qualitative only:
http://www.icas-proceedings.net/ICAS2008/PAPERS/073.PDF

The 747-8 is definitely better than the 747-400.

Quoting JAAlbert (Reply 5):
A very interesting statement ...., are you saying that the 787's glide ratio is better than any other commercial passenger plane in the skies?

Yes.

Quoting JAAlbert (Reply 5):
What makes it so? How much better? What exactly does that mean?

A combination of very high aspect ratio and the best aerodymamics we've got so far. My gut feeling is it's about 5% better, since Boeing was claiming about 1/4 of the 20% improvement on the 787 was due to aerodynamics, but that's an extremely rough measure.

Quoting JAAlbert (Reply 5):
The engines go out and the plane can fly another 200 miles or what?

On the order of another 7 miles or so (typical glide range for airliners is about 100-150 miles).

Quoting JAAlbert (Reply 5):
Also, how will the 350 compare to the 787?

I would expect it to be equal or better, for most of the same reasons as the 787's is better than most else that's out there.

Quoting JAAlbert (Reply 5):
What does this mean? The plane drops 1 ft every 20 ft? 1000ft every 20,000ft? That seems a pretty impressive glide ratio if so, no?

Yes, that's what it means. But the speeds are high so, as others have posted, you don't really have that long.

Something to keep in mind is that, although turbofans make quite a bit of thrust even at idle, it's only a tiny fraction of their rated thrust. So in a normal idle descent you're basically gliding. If you actually go engines dead your descent rate goes up a bit but it's not dramatic.

Tom.
 
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ER757
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RE: Could An A380 Glide?

Sat Jun 30, 2012 10:54 pm

Quoting tdscanuck (Reply 9):
Quoting BoeingGuy (Reply 4):
What's the 747-8 glide ratio? Do you know? Is it okay to post it, or is that also proprietary

As far as I know, it's proprietary.

When flight testing, are all engines purposely shut down at some time to test the glide ratio in the real world, or is it all done via computer simulation?

Quoting skychef747400 (Thread starter):
This is my first post after years of reading your posts

Welcome to a.net
 
WingedMigrator
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RE: Could An A380 Glide?

Sun Jul 01, 2012 12:16 am

Quoting rfields5421 (Reply 8):
The B767 and A330 were both at about 12:1

If that is true, the 777 can't have been at 20:1. It's simply not plausible that they would be that far apart.
 
rfields5421
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RE: Could An A380 Glide?

Sun Jul 01, 2012 12:37 am

Quoting WingedMigrator (Reply 11):
Quoting rfields5421 (Reply 8):The B767 and A330 were both at about 12:1
If that is true, the 777 can't have been at 20:1. It's simply not plausible that they would be that far apart.

I should have said that the near 12:1 was observed performance during an entire actual unpowered flight event.

Both the B767 and the A330 could have flown more miles. Both had to drop altitude and try to bleed off speed to make their landings. They had to hit the runway in front of them.

The A330 flew about 65 nm with no power from 33,000 feet and had to execute some 360 turns to descend enough to make the runway.

The B767 first indication of a problem came about 120 nm from the eventual landing site. The plane descended from 41K to 35K before the second engine quit. At one point in the flight, the plane was measured as descending 5,000 ft in 10 nautical miles - which works out to 12:1. Later in the descent the glide ratio improved a bit. They also realized they were too high when the runway came in sight. Rather than do a 360, the pilot forward slipped the aircraft to cause it to descend at a higher rate and not pickup speed.

Glide ratio is always a bit theoretical and no jetliner aircraft is expected to achieve a perfect glide ration from start to end of an unpowered event. Something which hurts the overall Glide Ration would be the realtively high speed at cruise when the event begins - the aircraft has to slow to its best glide speed before it begins to achieve its best possible glide ration.
 
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czbbflier
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RE: Could An A380 Glide?

Sun Jul 01, 2012 1:53 am

Quoting skychef747400 (Thread starter):
After watching documentaries on Gimli Glider and Air Transat 236 I have been wondering if a 380 was ever in that suituation god forbid where there was no engines would it still be able to glide.

As you can see, the answer is definitively "Yes".

However, this will apparently not be an issue so long as there are no Canadian registered A-380s. Seems it's only Canadian registered airliners that run out of gas part-way through its flight.

But once one is registered in Canada, give it a few years and we'll soon find out!







(kidding)
 
AngMoh
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RE: Could An A380 Glide?

Sun Jul 01, 2012 2:20 am

Quoting czbbflier (Reply 13):
As you can see, the answer is definitively "Yes".

However, this will apparently not be an issue so long as there are no Canadian registered A-380s. Seems it's only Canadian registered airliners that run out of gas part-way through its flight.

But once one is registered in Canada, give it a few years and we'll soon find out!

To be fair, the Germans can also do it - it is not only the Canadians:

http://en.wikipedia.org/wiki/Hapag-Lloyd_Flight_3378
 
tdscanuck
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RE: Could An A380 Glide?

Sun Jul 01, 2012 2:29 am

Quoting ER757 (Reply 10):
When flight testing, are all engines purposely shut down at some time to test the glide ratio in the real world

Not that I'm aware of, at least for jet airliners. There is no circumstance where you shut down both engines (you shut down one pretty frequently). There are less risky and just as accurate ways to find the relevant aerodynamic parameters.

Quoting ER757 (Reply 10):
is it all done via computer simulation?

There will certainly be a projection from the simulations but the wings don't know or care if the engines are running or not, they just see air flowing over them. As long as you have accurate L/D information (which you get from flight testing) then you know what the glide ratio is without actually having to do a real unpowered glide.

The whole point of many flight test maneuvers like wind-up turns and roller coasters is to get the wings into odd aerodynamic situations so you can extract the true performance parameters.

Tom.
 
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zeke
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RE: Could An A380 Glide?

Sun Jul 01, 2012 4:02 am

Quoting skychef747400 (Thread starter):
After watching documentaries on Gimli Glider and Air Transat 236 I have been wondering if a 380 was ever in that situation god forbid where there was no engines would it still be able to glide.

Almost every jet aircraft that descends, does so with idle thrust, they are effectively gliding from top of descent every flight.

Quoting rfields5421 (Reply 8):

Yes - 1 ft down for every 20 feet forward - assuming a constant speed.

The 777 "TWO ENGINE INOPERATIVE DRIFTDOWN" table shows from FL400 it will do 125 nm at 198 KIAS, that is less 20:1 at the optimum L/D speed. At 777 and 270 kt, it is similar to the A330 at 300kt, 106 nm.

Quoting rfields5421 (Reply 8):

The B767 and A330 were both at about 12:1

The A330 is better than that, looking at the all engines flame out QRH procedure, the aircraft will do 100 nm from FL400 (about 15:1) using a fixed speed strategy of 0.82/300kt. You would do better green dot, it is better than 3nm per 1000 ft.
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grimey
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RE: Could An A380 Glide?

Sun Jul 01, 2012 4:46 am

Anyone know what the BA 9 glide ratio was since that was on a B747-400, I was trying to find some information on that but came across this link on Wikipedia

http://en.wikipedia.org/wiki/List_of...line_flights_that_required_gliding

What about the BA B777-200 (BA 38) that had to do a slight glide in LHR, although the captain changed the flap setting from 30 to 25 degrees in order to reduce drag and increase the glide.

Quoting BoeingGuy (Reply 2):
Older tail mounted airplanes like the 727 apparently have a significantly lower glide ratio.

From what I understand is that with the tail being pushed down due to the extra weight at the rear, then the Angle of Attack is increased and therefore it is harder to maintain a good glide ratio.
 
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RE: Could An A380 Glide?

Sun Jul 01, 2012 5:07 am

Quoting grimey (Reply 17):
Quoting BoeingGuy (Reply 2):Older tail mounted airplanes like the 727 apparently have a significantly lower glide ratio.
From what I understand is that with the tail being pushed down due to the extra weight at the rear, then the Angle of Attack is increased and therefore it is harder to maintain a good glide ratio.

A more accurate explantion is that the 727, like most airplanes of its era, had lower aspect ratio wings than today's jet transports. Its best L/D was around 14 compared to around 20 for a modern design.
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BoeingGuy
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RE: Could An A380 Glide?

Sun Jul 01, 2012 7:11 am

Quoting rfields5421 (Reply 12):
The A330 flew about 65 nm with no power from 33,000 feet and had to execute some 360 turns to descend enough to make the runway.

Actually it was about 90 nm IIRC.

Quoting tdscanuck (Reply 9):
Quoting JAAlbert (Reply 5):
The engines go out and the plane can fly another 200 miles or what?

On the order of another 7 miles or so (typical glide range for airliners is about 100-150 miles).

I think it's more like 90 miles. The quick rule of thumb is about 2 miles for every 1000 feet altitude. You certainly don't want pilots pulling out the charts in that situation, so the 2 miles for 1000 feet thing is just a quick estimate. The A330 showed they could beat that. A 777 or 787 would most certainly beat that - probably significantly for the 787. However, you really don't want to ever have to test that efficiency. If all engines failed, you'd much rather have a choice of easily reachable airports below you rather than getting to be the first crew to find out just how far a 787 can glide before hitting the ground (or water).
 
speedbird128
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RE: Could An A380 Glide?

Sun Jul 01, 2012 8:13 am

Quoting skychef747400 (Thread starter):
be able to glide

Usually every descent is made at thrust idle, so gliding is done routinely by every aircraft, A380 not excluded.

Some just have a better glide ration than others. Simply put its the rate of going forwards versus the rate of going down.

I would imagine the A380 has a reasonably good glide ratio.
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RE: Could An A380 Glide?

Sun Jul 01, 2012 8:26 am

Quoting skychef747400 (Thread starter):
This is my first post after years of reading your posts, opinions and information

Prepare for everything you hold dear about the civil aviation industry (favourite airline, airport, cabin crew etc) to be blown out of the water by the largest torpedo you have ever set eyes upon.

It is a simmering bubbling cauldron of open warfare.

We make sports fans look like pussy cats.

(but in a fun way)

Countries, governments, aircraft manufacturers.....none escape the onslaught.

  

(lol)
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RickNRoll
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RE: Could An A380 Glide?

Sun Jul 01, 2012 8:40 am

I have read one description of the A380 that said it was essentially a 'self launching glider'. When you see them fly, you can see why. They just seem to float in the air.
 
Rheinbote
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RE: Could An A380 Glide?

Sun Jul 01, 2012 9:06 am

Both the 787 and the A350 have an L/D of ~21:1. This is amongst others enabled by the use of CFRP for the primary wing structure, which enable aspect ratios beyond the maximum 9...10 achievable with metal wings [large commercial airliner context, ~M0,85]

A380 should have a lower L/D, I'd guess around 18...19 - this is not a matter of inferior technology, it is the result of a design compromise. First, the wing had to stay within the 80m span limit, so an otherwise achievable aspect ratio of 9...10 was out of question. Second, the A380 had to make use of four engines, as even the two most powerful engines available wouldn't have provided enough thrust. In a glide, four engines have more drag than two and a lower aspect ratio wing is less efficient.

On the other hand, the A380's low-set wing has a pronounced ground effect, so an unpowered landing would to be nicely cushioned.  
 
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AirlineCritic
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RE: Could An A380 Glide?

Sun Jul 01, 2012 6:03 pm

Quoting BoeingGuy (Reply 19):
If all engines failed, you'd much rather have a choice of easily reachable airports below you rather than getting to be the first crew to find out just how far a 787 can glide before hitting the ground (or water).

I'm curious about the spacing of airports in the world. If we take 11 kilometers as the default cruise altitude, and 787's glide ratio as 21, then we get a glide range of roughly 230 kilometers. Are there areas in, say, US or EU where this would not be enough to reach a commercial airport? How about any airfield?

Obviously over sea 230 kilometers may not be enough. Anyway, hopefully no one has to experience running out of glide range in ocean or elsewhere.
 
rfields5421
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RE: Could An A380 Glide?

Sun Jul 01, 2012 7:09 pm

Quoting AirlineCritic (Reply 24):
Are there areas in, say, US or EU where this would not be enough to reach a commercial airport? How about any airfield?

US - Certain regions of the Rocky Mountains might fit that criteria - though airport elevations being close to 5,000 ft in places - that can cut the glide range considerably.

Alaska - obviously many areas that would be out of range.
 
prebennorholm
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RE: Could An A380 Glide?

Mon Jul 02, 2012 3:03 am

Quoting AirlineCritic (Reply 24):
Are there areas in, say, US or EU where this would not be enough to reach a commercial airport?

Unfortunately total power loss is more likely to happen at lower altitude. Three events in modern time in the US and EU didn't happen at anything like FL300-400, but at 3000 feet (ARN 1991), 700 feet (LHR 2008) and 3000 feet (JFK 2009). None made it to an airport. By pure luck no fatalities, but a few dozen injuries, two or them serious.

The "Gimli Glider" in Canada (1983) made it to a closed military air base after gliding from FL410. Anyway it wasn't without drama. The pilot couldn't calculate the landing properly since missing hydraulic pressure on the B767 he could not extend flaps and slats, or spoilers for glide angle control. Consequently it came in way to long and fast. Also lack of hydraulic pressure prevented the nose wheel to lock, so it came back up as soon as touching the runway. The plane was difficult to control in gliding flight since the RAT (ram air turbine) at the low gliding speed delivered too little power.

Wiki writes about that incident: Several attempts by other crews who were given the same circumstances in a simulator at Vancouver resulted in simulated crashes.

So please keep those engines spinning, and please don't test those L/D ratios.
Always keep your number of landings equal to your number of take-offs
 
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DocLightning
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RE: Could An A380 Glide?

Mon Jul 02, 2012 4:52 am

Quoting tdscanuck (Reply 9):
As far as I know, it's proprietary. There's a very good public paper on the 747-8 development that includes a discussion of L/D changes on the 747-8 but it's qualitative only:

Why wouldn't that be published data? Doesn't a crew need to know that if they have an all-flame-out situation?
-Doc Lightning-

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tdscanuck
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RE: Could An A380 Glide?

Mon Jul 02, 2012 11:15 am

Quoting DocLightning (Reply 27):
Why wouldn't that be published data?

A couple of reasons. For starters, it's basically a direct measure of how good the aerodynamics are, which the OEM's don't like to share for the same reason they don't like sharing exact fuel burn numbers; it's too easy for the competition and the public to use/abuse/screw-up a one-number value.

Also, the lift/drag polar is really a huge family of curves by weight, speed, configuration, AoA, and some other things. A single value (max L/D) is useful in one and only one circumstance...gliding at best glide speed. Gliding at all is pretty rare, gliding at best glide speed almost never happens.

Quoting DocLightning (Reply 27):
Doesn't a crew need to know that if they have an all-flame-out situation?

They get the equivalent data in a much more useful form; they get a target speed. That will get them however much range they can get for the current weight/altitude. That's far more practical than trying to navigate a L/D vs. weight vs. configuration vs. speed table while simultaneously trying to call for help and get the engines restarted.

Tom.
 
BoeingGuy
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RE: Could An A380 Glide?

Mon Jul 02, 2012 3:04 pm

Quoting prebennorholm (Reply 26):
The "Gimli Glider" in Canada (1983) made it to a closed military air base after gliding from FL410. Anyway it wasn't without drama. The pilot couldn't calculate the landing properly since missing hydraulic pressure on the B767 he could not extend flaps and slats, or spoilers for glide angle control. Consequently it came in way to long and fast. Also lack of hydraulic pressure prevented the nose wheel to lock, so it came back up as soon as touching the runway. The plane was difficult to control in gliding flight since the RAT (ram air turbine) at the low gliding speed delivered too little power.

That's not totally correct. The reason he didn't extend the speedbrakes is because the RAT only powers the Center Hydraulic System. The C Hyd system does power some of the individual spoiler panels, but not symmetrical ones on each side on the 767. Thus he chose not to extend them. He put the airplane in a major sideslip to bleed off speed.

That's not why the nose wheel gear collapsed. There is an alternate gear extension if necessary. An airplane gliding on RAT power can extend and lock the landing gear just fine. It collapsed because a concrete barrier had been built across the closed runway and the nosewheel gear hit it.
 
prebennorholm
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RE: Could An A380 Glide?

Tue Jul 03, 2012 12:47 am

Quoting BoeingGuy (Reply 29):
That's not why the nose wheel gear collapsed. There is an alternate gear extension if necessary. An airplane gliding on RAT power can extend and lock the landing gear just fine. It collapsed because a concrete barrier had been built across the closed runway and the nosewheel gear hit it.

Well, it's a long time ago, but that's not how I remember it. And also not how Wiki describes it today: From Wiki:

Without power, the pilots had to try lowering the aircraft's main landing gear via a gravity drop, but, due to the airflow, the nose wheel failed to lock into position.

As soon as the wheels touched the runway, Pearson "stood on the brakes", blowing out two of the aircraft's tires. The unlocked nose wheel collapsed and was forced back into its well, causing the aircraft's nose to scrape along the ground.


But you are right, captain Pearson successfully used sideslip to slow down speed, something he was familiar with as a keen spare time glider pilot.

But dear BoeingGuy, you are definitely wrong about the "concrete barrier had been built across the closed runway".
The barrier was parallel to the runway - in the middle, dividing the runway into two race car tracks.
It was not concrete, but an ordinary metal roadside barrier.
You can see that on several pictures on the net, including Wiki. Just google "gimli glider wiki". And pictures remember well. That doesn't exclude that it could have played a role in the nose gear collapse, even if Wiki and my memory says otherwise.

Otherwise I am willing to accept that my memory over almost three decades can be rusty - and that Wiki too often isn't accurate in all details.

And thanks for the details about the 767 hydraulic limitations when operated on RAT.

Anyway, my main message still stands. A dead stick landing is a lot more than just reaching the an airport. The Gimli Glider incident proves that. It could easily have become a terrible disaster. Planes are simply not designed for that stunt.

[Edited 2012-07-02 18:11:26]
Always keep your number of landings equal to your number of take-offs
 
KELPkid
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RE: Could An A380 Glide?

Tue Jul 03, 2012 12:56 am

Quoting DocLightning (Reply 27):
Why wouldn't that be published data? Doesn't a crew need to know that if they have an all-flame-out situation?

Vg (the best rate of glide speed) is the speed that you want to trim the aircraft for (ideally) in an engine-out situation. It is also coincident with L/Dmax. I know that, historically, since about 1980 or so, Boeing has never published the best rate of glide speeds, as an all engine out situation is considered "statistically unlikely."   Guess (unprofessional) boneheads don't exist in this world...except at Gimli or the Azores or the odd 747 crew who flys through volcanic ash clouds   Wonder if Airbus publishes best rate of glide speeds.

Of course, that number probably varies wildly in an airliner, depending on CG and airframe configuration issues (like the flap and slat settings, gear up or down, etc.).
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prebennorholm
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RE: Could An A380 Glide?

Tue Jul 03, 2012 1:43 am

Quoting KELPkid (Reply 31):
Of course, that number probably varies wildly in an airliner, depending on CG and airframe configuration issues

Sure you are right.

But when it is about dead sticking into a far away airport, then the biggest variable is the wind. At altitude we can easily have 100 mph headwind or tailwind, giving a 200 mph difference in ground speed at the same vertical descend speed.

To cover the most ground in headwind one should speed up somewhat above best L/D speed to get the best glide ratio over ground, and slow down at tailwind.

In a theoretical case it may mean that the nearest airport is unreachable because it is upwind, while a downwind airport slightly further away is reachable.
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r2rho
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RE: Could An A380 Glide?

Thu Jul 19, 2012 12:36 pm

The theoretical calculation is:

tan(gamma) = D/L (or inverse of L/D) with gamma being the angle between glide path and ground.

Or conversely, if you know the height from which an aircraft glides, and the distance covered, you can find out gamma, from which you can find out L/D ratio. So if you want to get the A380's L/D, just ask Airbus flight test to do an all-engine out glide from a designated altitude at best glide speed and tell you how far they got  

You can see that for best glide, you want the best possible L/D. Note that this does not necessarily mean minimum vertical speed.
 
wingscrubber
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RE: Could An A380 Glide?

Fri Aug 10, 2012 5:09 pm

Related thread:
Could An A380 Glide After Loss Of All Engine Power (by Bcal Jan 28 2005 in Civil Aviation)

Here, somebody estimates a glide ratio of 20:1;
http://answers.yahoo.com/question/index?qid=20090116072155AAkgRZp

This is a good example of a large 4-engined widebody successfully gliding long enough to achieve engine restart:
http://en.wikipedia.org/wiki/British_Airways_Flight_9

In summary, the answer is: yes.
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musang
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RE: Could An A380 Glide?

Wed Aug 15, 2012 3:38 pm

Quoting grimey (Reply 17):
Anyone know what the BA 9 glide ratio was since that was on a B747-400,

747-200, Rolls Royce, IIRC.

Quoting prebennorholm (Reply 26):
700 feet (LHR 2008) and 3000 feet (JFK 2009). None made it to an airport.

The Heathrow one did!

Another complication so far not raised is the need, immediately after total engine loss, to keep the airspeed high and the engines windmilling faster, thereby improving the chances of a restart. If there's no fuel left, this is irrelevant of course, and one needs to fly the best glide speed immediately.

If the engines slow down below a specified RPM, they have to be electrically or APU air-started according to type. At high levels the APU may not start.

Therefore priority one is to pitch down (its not that dramatic an angle) and aim, if you're in a 737 classic, for 275 knots indicated if you're above flight level 270. Below FL270, fly 300 knots. Windmilling the engines is a higher priority at this stage than best glide speed.

When you decide no engines will restart, fly 220 knots and expect to cover approx 2 miles for each 1000 feet, corrected for wind. Worst case scenario (in the sim of course) is that the APU won't start and we give up windmilling at, say, 15,000 feet so as to concentrate on the landing. So from the cruise, only the last 15,000 feet in this case is flown at best L/D or something approximating it.

Aerodynamicists -

1) Does the best L/D speed vary with weight and C of G?

2) Is the best holding speed from the FMC a good approximation of best L/D?

3) How would one determine the best glide speed for distance as opposed to the best speed for endurance/time?

Regards - musang
 
tdscanuck
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RE: Could An A380 Glide?

Wed Aug 15, 2012 5:19 pm

Quoting musang (Reply 35):
At high levels the APU may not start.

Although true, this isn't supposed to be as much of an issue as it used to be. The starting requirements on APUs have gotten stricter.

Quoting musang (Reply 35):
1) Does the best L/D speed vary with weight and C of G?

Yes.

Quoting musang (Reply 35):
2) Is the best holding speed from the FMC a good approximation of best L/D?

It should be close. Hold speed should be min fuel consumption (which won't be min drag but should be close).

Quoting musang (Reply 35):
3) How would one determine the best glide speed for distance as opposed to the best speed for endurance/time?

Best L/D gives you best distance. Best speed for endurance/time would be that which gives you minimum descent rate...I think this would also be the same speed in a glide.

Tom.

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