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Fear Of Flight & Redundency In Flight Control?  
User currently offlinePs76 From , joined Dec 1969, posts, RR:
Posted (8 years 10 months 1 week 6 days 8 hours ago) and read 4519 times:

Hi

I'll admit flying makes me fearful - one of the aspect I worry most on (besides unbeleivably how the aircraft structure might fall apart/I freefall from 30000ft!) is flight controls. Is there any good in-depth sources of info on flight controls/redundency that might help put my mind at rest?

Read today the 777 has fly-by-wire but also mechanical operation if there's a total electrical failure. Made me feel better knowing that even though I don't think I've ever heard of a total electrical failure ever happening. Do Airbii(?) also have this feature or what other general practices are used for redundency (independent aileron/flaperon systems?/multiple hydraulic actuators on their own electrical circuits?). I'm a total beginner in this field so any info would be great.

Also, do most non fly-by-wire aircraft really use cables/pulleys as the only mechanism? How many backups do they have & how would they work? I'm always fearful that the cable might snag/break with no backup even though I've never heard of it happening in an airliner. However, I would also be nervous about most light aircraft if basically every control surface was dependent on only 1 cable like my mountain-bike brakes with no backup (though I'm sure maintained much better than my mountain-bike!). If it really is only one 'it works or big problems' cable for roll/pitch/yaw I might feel better with more direct 'stick & rudder' (again it's the snag thing) though again my fear is probably not based on much/any fact!

Any info on basically why I don't need to fear the ailerons/elevator/rudder uncontrollably turning the aircraft upside-down would be great.

Thanks


P.

23 replies: All unread, jump to last
 
User currently offlineMrChips From Canada, joined Mar 2005, 927 posts, RR: 0
Reply 1, posted (8 years 10 months 1 week 6 days 7 hours ago) and read 4506 times:

OK...total flight control failure is extremely rare - you can count the number of instances of this on one hand (in modern aircraft, that is).

In any fly-by-wire aircraft, there will be three (or more) completely independent computers and circuits that can control the aircraft, each of which on their own have the ability to safely fly the aircraft. Even in the event of all of these systems failing, there is a manual reversion system that gives enough control to fly the aircraft to a safe landing.

Now most non-FBW transport aircraft use pulleys, cables and pushrods to actuate the control surfaces via hydraulic actuators. There generally isn't a backup for the cables and pushrods, but there are a couple of things that prevent total failure:

1) independent flight control systems - if necessary, the two pilots can de-couple their controls and fly the aircraft from one set only.

2) multiple redundant and independent hydraulic systems - this is present in FBW aircraft as well, but if one hydraulic system fails, then the others should be able to get the aircraft on the ground safely. However, there are two very prominent instances of total hydraulic failure, that of United 232 and Japan Airlines 123, in which a catastrophic failure cut lines from all of the hydraulic systems.

3) manual reversion - not the best way to fly, but it can be done.

In a non-hydraulic aircraft (basically, anything smaller than a DHC-8), if there was a cable/pulley/pushrod failure, then it may be possible to control the aircraft using trim settings as well as by the application or removal of power. This ability also exists in the other control protocols as well.



Time...to un-pimp...ze auto!
User currently offlinePilotpip From United States of America, joined Sep 2003, 3144 posts, RR: 11
Reply 2, posted (8 years 10 months 1 week 6 days 7 hours ago) and read 4498 times:

As far as the cable systems, there is usually redundancy built into them through a "split control" system. IE: The copilot's yoke and cables control one of the ailerons and the captain's controls the other side. This system is equipped with some sort of clutch that disengages the two systems in case one becomes jammed and they will work independently. Also, there are other means of controlling attitude. For example, if your ailerons fail you can control your turn rate by usage of the rudder and in a multi-engine you can use differential power.

Most transport category aircraft have at least two of everything, and in some cases three of everything for the sake of redundancy. This exponentially reduces the likely hood of a catastrophic event if one fails, but it will never eliminate the possibility. Given the number of aircraft flying the number of people every day you can see that these events nearly never happen. Also, if one thing fails there are other instruments and controls that will result in the pilot getting the same information and/or aircraft response.

Hydraulic and cable control systems are usually routed through a number of different places to prevent all systems being knocked out by some sort of damage. The UAL DC-10 back in the day was a prime example of what happens when all three systems are knocked out. Capt. Haynes and the guys did a good job with what they had to control the aircraft. They used differential power to control rate of descent and make the only turns they could make to the left. Their work saved many lives on that flight.

I have hundreds of hours in small aircraft and have never had a control issue (I've never broken a cable on my bicycles either). They're quite simple and strong which is why these, or steel pushrods are the preferred method of actuation. While not as redundant as large aircraft there are inspections done regularly and a proper preflight will tell you if all controls are working properly. Even if one did get stuck, I would know what to do using what I had available to get the aircraft on the ground. It may not be pretty but I'm pretty confident that I could walk away from that. We often practice no flap landings and set up situations where we simulate a control failure to practice such situations.

Oh, and this fourm is a great place to get info that will hopefully help you tame those fears. I often find that knowledge can help with things I'm not comfortable with or don't understand.



DMI
User currently offlineHaveBlue From United States of America, joined Jan 2004, 2098 posts, RR: 1
Reply 3, posted (8 years 10 months 1 week 6 days 3 hours ago) and read 4465 times:

Quoting Pilotpip (Reply 2):
They used differential power to control rate of descent and make the only turns they could make to the left.

Not to be a nitpicking prick, but they were only able to make right turns, not left. Just so happens I read a lecture by Capt. Al Haynes just 2 days ago about the whole incident.

 Smile



Here Here for Severe Clear!
User currently offlineJush From Germany, joined Apr 2005, 1636 posts, RR: 3
Reply 4, posted (8 years 10 months 1 week 6 days 3 hours ago) and read 4457 times:

Quoting HaveBlue (Reply 3):
Not to be a nitpicking prick, but they were only able to make right turns, not left. Just so happens I read a lecture by Capt. Al Haynes just 2 days ago about the whole incident.

That Siox City (excuse my knowledge of writing that city name) incident?
That was amazing stuff. Saw a documentary on it once.

Regards
jush



There is one problem with airbus. Though their products are engineering marvels they lack passion, completely.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 5, posted (8 years 10 months 1 week 5 days 21 hours ago) and read 4417 times:

Quoting Ps76 (Thread starter):
I'm always fearful that the cable might snag/break with no backup even though I've never heard of it happening in an airliner.



Quoting Pilotpip (Reply 2):
have hundreds of hours in small aircraft and have never had a control issue (I've never broken a cable on my bicycles either)

As you can read, even from your own comments, total failure of the flight controls just doesn't happen. It's so astronomically unlikely. Even the Sioux City DC-10 had some control left, and that accident lead to big design criteria changes.

Certification regs state that even if everything used in normal operation breaks, there should be at least one backup.

If you want to worry about something, worry about human factors (maintenance, piloting, etc...) They're much more likely to lead to an accident than mechanical issues. Ok so maybe I'm not helping your fear of flying here  Wink



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2543 posts, RR: 25
Reply 6, posted (8 years 10 months 1 week 5 days 19 hours ago) and read 4388 times:

Redundancy in aircraft design is such that catastrophic failure is extremely unlikely. With flight controls there is always a get you home scenario. Sioux City was a one off where even that scenario failed, yet the crew managed to keep control by using human ingenuity. Most flying accidents are the result of a combination of factors, with human error usually being part of the chain.

From my personal experience, the best way to overcome a fear of flying is to fly often, especially with someone sympathetic who does not share your fear. After a while it's like getting on a bus. You then realise how much safer it is than driving yourself around in your car.

Also I'd recommend taking a trial flying lesson. And of course reading and posting on this forum.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 7, posted (8 years 10 months 1 week 5 days 18 hours ago) and read 4378 times:

As the good posts above have stated, the systems are safe against just about any expected, and some unexpected failures. Still, flight control failures have happened.

There are airshow acts out there (I expect to see it at Reno this weekend) where the pilot actually drops one aileron off the airplane, then does an aerobatic routine without it. On a related matter I saw a big piece come off a P-51 Mustang yesterday. He was making an emergency landing and on final approach I thought he jettisoned the canopy but it was some large part that fell off, he still landed just fine. Not too surprising when things like that happen - the plane is sixty years old and was going a hundred knots faster than it was designed for.

I lost rudder control once due to battle damage, and had no real trouble landing. On another occasion I got uncommanded roll spoiler deployments. I survived that, and my passengers never even knew. (They just thought I was a lousy pilot.)

A friend of mine was doing stalls in an older Cessna single which someone had modified. In recovery from the stall the aileron control (a bicycle chain between the two yokes) derailed itself over a misrouted piece of aluminum tubing. This froze his ailerons and elevators, leaving him only with rudder and with elevator trim - which now worked in reverse. He flew around for a while playing with what control he had, and eventually landed the plane without further incident.

Another old friend of mine had a pitch horn break on a Bell Jetranger. One main rotor blade feathered while the other was still controllable. An unsurvivable situation but he did get it back on the ground safely.

The posters above are correct. The tiny bit of risk you do take will probably come at you from a different cause. I really don't like flight control problems but the level of redundancy is actually quite good there.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 8, posted (8 years 10 months 1 week 5 days 16 hours ago) and read 4358 times:

Quoting MrChips (Reply 1):
In any fly-by-wire aircraft, there will be three (or more) completely independent computers and circuits that can control the aircraft, each of which on their own have the ability to safely fly the aircraft.

Compleletly independent can be misleading. The computers communicate with each other, share data and power sources. They are, however, capable of operating independently and their reactions to failures of all shared resources are scrupulously studied to ensure that the system as a whole meets its safety objectives.

Quoting MrChips (Reply 1):
Even in the event of all of these systems failing, there is a manual reversion system that gives enough control to fly the aircraft to a safe landing.

Not always. Though the 777 and most Airbus can (theoretically) be flown without the FBW system, the A340-600 cannot.

In all Airbuses up to the 346, the rudder is commanded through cables, as is the stabilizer trim (although the FBW system can move the trim as well). Theoretically, the plane is controllable with these surfaces.

The 777 has one pair of roll spoilers and the stabilizer trim that can be commanded indepentently from the FBW system. Again theoretically, the plane is controllable with these surfaces.

In practice, many operational conditions (wind, turbulence, engine failure) can render these planes uncontrollable with only those surfaces. Still, these planes meet their safety objectives without taking credit for this 'last stand' mode.

What are the safety objectives? Well, each failure condition capable of generating a catastrophe (i.e. loss of control) has to be shown to be extremely improbable. In certification, this means a probability of occurance lower than one in one billion flight hours. Additionally, no single failure may generate this condition, no combination of a single failure with latent failures (failures not immediately aparent, that could remain undetected during several flights) can cause this condition. No single failure combined with a probable failure (a failure more likely than one in one hundred thousand flight hours) may cause this condition.

As you can see, flight control systems are designed to be extremely robust.

mrocktor


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 9, posted (8 years 10 months 1 week 5 days 15 hours ago) and read 4341 times:

Quoting Mrocktor (Reply 8):
Quoting MrChips (Reply 1):
Even in the event of all of these systems failing, there is a manual reversion system that gives enough control to fly the aircraft to a safe landing.

Not always. Though the 777 and most Airbus can (theoretically) be flown without the FBW system, the A340-600 cannot.

In all Airbuses up to the 346, the rudder is commanded through cables, as is the stabilizer trim (although the FBW system can move the trim as well). Theoretically, the plane is controllable with these surfaces.

The 777 has one pair of roll spoilers and the stabilizer trim that can be commanded indepentently from the FBW system. Again theoretically, the plane is controllable with these surfaces.

As Mrocktor points out, actually getting to the "last stand" is pretty tricky. Multiple independent and extremely unlikely failures have to occur before you even get close. The likelihood of the pilots screwing up before then is much higher Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (8 years 10 months 1 week 5 days 15 hours ago) and read 4340 times:

However . . .

I have been told that if the primary flight guidance system (whatever they call it) failed in the MD-11 there is no way to fly the airplane at all. If you were VMC over an airport you could not make it go there.

Anyone know if that is true?



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineGigneil From United States of America, joined Nov 2002, 16347 posts, RR: 85
Reply 11, posted (8 years 10 months 1 week 5 days 14 hours ago) and read 4327 times:

Quoting Mrocktor (Reply 8):
In all Airbuses up to the 346, the rudder is commanded through cables, as is the stabilizer trim (although the FBW system can move the trim as well). Theoretically, the plane is controllable with these surfaces.

And every A340 and A330 built since the first QF 332. All production 330/340 series planes feature the new FBW system and rudder controls.

Newly built A320 series planes since the A318 also feature the revised system, if I'm not mistaken.

N


User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 12, posted (8 years 10 months 1 week 4 days 16 hours ago) and read 4243 times:

Quoting Gigneil (Reply 11):
And every A340 and A330 built since the first QF 332. All production 330/340 series planes feature the new FBW system and rudder controls.

Newly built A320 series planes since the A318 also feature the revised system, if I'm not mistaken.

Wow! I didn't know that. Interesting that the modification was judged to be so valuable as to be rolled back to all families and variants. As far as I know it is not only a matter of pulling the cables and putting in the FBW system for the rudder: some changes were made to the electrical system as well.

I know they did the modification on the A340-600 because the conventional system was not viable (not sure if it was because of the length of the cables or if the airplane's yaw dynamics required active control beyond the capabilities of a simple yaw damper). That the benefit was large enough to merit widespread application is somewhat surprising.

mrocktor


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 13, posted (8 years 10 months 1 week 4 days 12 hours ago) and read 4211 times:

Quoting Mrocktor (Reply 12):
I know they did the modification on the A340-600 because the conventional system was not viable (not sure if it was because of the length of the cables or if the airplane's yaw dynamics required active control beyond the capabilities of a simple yaw damper). That the benefit was large enough to merit widespread application is somewhat surprising.

Well, the 346 is so long that the flight control system actually has to move the elevator to relieve stress on the airframe. So I imagine you are correct on the other thing, although it could just be the almighty dollar talking.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineVSIVARIES From United Kingdom, joined Aug 2004, 108 posts, RR: 4
Reply 14, posted (8 years 10 months 1 week 4 days 11 hours ago) and read 4197 times:

Erm hold on a minute Slam,

Quoting SlamClick (Reply 7):



Quoting SlamClick (Reply 7):
I saw a big piece come off a P-51 Mustang yesterday



Quoting SlamClick (Reply 7):
I lost rudder control once due to battle damage



Quoting SlamClick (Reply 7):
On another occasion I got uncommanded roll spoiler deployments



Quoting SlamClick (Reply 7):
the aileron control (a bicycle chain between the two yokes) derailed itself over a misrouted piece of aluminum tubing. This froze his ailerons and elevators



Quoting SlamClick (Reply 7):
Another old friend of mine had a pitch horn break on a Bell Jetranger

Did you not read the original post of our friend Ps76?

Quoting Ps76 (Thread starter):
I'll admit flying makes me fearful - one of the aspect I worry most on (besides unbeleivably how the aircraft structure might fall apart/I freefall from 30000ft!) is flight controls

 confused  Blimey, I realise you have been flying planes since just after they were invented  duck  but you've nearly put me off flying (and I'm training to be a pilot!). What about all the statistical bullshi;t, surely that is what we should be poinint out to Ps76?

(As I reach for my flameproof suit!)  spin 

B/R



For every action there is always an unequal but mostly similar reaction.
User currently offlinePs76 From , joined Dec 1969, posts, RR:
Reply 15, posted (8 years 10 months 1 week 4 days 10 hours ago) and read 4188 times:

Hi

Thanks for the info - for me hopefully the more I learn the less fearful I'll feel (it's pretty interesting also).

Another problem I have is pressurization & how it's run at 30000ft. - though maybe for another time/post..

Thanks


P.


User currently offline777WT From United States of America, joined Jan 2005, 875 posts, RR: 1
Reply 16, posted (8 years 10 months 1 week 4 days 8 hours ago) and read 4179 times:

Quoting Ps76 (Reply 15):
Another problem I have is pressurization & how it's run at 30000ft. - though maybe for another time/post..

Bleed air  Wink


User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 17, posted (8 years 10 months 1 week 3 days 21 hours ago) and read 4147 times:

Quoting Ps76 (Reply 15):
though maybe for another time/post..

By all means do another post, we'll be happy to comment on it and it will keep the forum organized  Smile

mrocktor


User currently offlineStarlionblue From Greenland, joined Feb 2004, 16991 posts, RR: 67
Reply 18, posted (8 years 10 months 1 week 3 days 9 hours ago) and read 4101 times:

Quoting Ps76 (Reply 15):

Another problem I have is pressurization & how it's run at 30000ft. - though maybe for another time/post..

Well, from a redundancy point of view, you have the masks. And no, small holes don't mean all the air is sucked out in 2 seconds like in the movies.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineHAWK21M From India, joined Jan 2001, 31667 posts, RR: 56
Reply 19, posted (8 years 10 months 1 week 3 days 1 hour ago) and read 4078 times:

Quoting Ps76 (Reply 15):
Another problem I have is pressurization & how it's run at 30000ft

http://www.b737.org.uk/pressurisation.htm
regds
MEL



Think of the brighter side!
User currently onlineMD11Engineer From Germany, joined Oct 2003, 13967 posts, RR: 63
Reply 20, posted (8 years 10 months 1 week 23 hours ago) and read 4007 times:

Quoting SlamClick (Reply 10):
However . . .

I have been told that if the primary flight guidance system (whatever they call it) failed in the MD-11 there is no way to fly the airplane at all. If you were VMC over an airport you could not make it go there.

Anyone know if that is true?

What do you mean with primary flight guidance system? I don't understand your question, else I might be able to answer it.

The MD-11 is basically a DC-10 with an A340 cockpit. Flight control is "fly by cable", including the spoilers. The primary flight control actuators contain additional valves, which are controlled by the two FCCs for autopilot input, of which each has two independent channels, one controlling and the other one monitoring. Each FCC receives attitude input from it's own IRU, plus one IRU as a hot spare.
You also have the traditional standby instruments, ADI with localizer and glideslope needles and Airspeed/altitude indicator, plus a magnetic compass. The VOR and ILS receivers controlling the standby instruments receiver power from the left emergency bus, which is powered in an emergency from the aircraft battery.

Jan


User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 21, posted (8 years 10 months 1 week 20 hours ago) and read 3990 times:

Quoting MD11Engineer (Reply 20):
You also have the traditional standby instruments, ADI with localizer and glideslope needles and Airspeed/altitude indicator, plus a magnetic compass. The VOR and ILS receivers controlling the standby instruments receiver power from the left emergency bus, which is powered in an emergency from the aircraft battery.

Hi Jan, Can't say about your MD-11s but ours (and any other jet I've flown) have no ILS capabilities through the standby flight instruments. The STDBY Att. ind is powered through the LEAC but no ILS.
Strictly off the batt. will get you 15-30 min of power. With the ADG deployed in HYD mode the ADG will power the LEAC and get you about 90 min of power. The ADG in ELEC mode will give you unlimited electrics as the batt. is now charged. We have no ILS untill this level of elec. pwr.

Don't forget to add that the closed cable flight control system is hydraulically actuated and each flight control has hydraulic redundancy.


User currently offlineBsergonomics From United Kingdom, joined Jan 2002, 462 posts, RR: 0
Reply 22, posted (8 years 10 months 5 days 8 hours ago) and read 3910 times:

In the military world, our hazard analyses have to demonstrate that, due to failures in the aircraft, the probability of a hull loss is less than one per million flight hours. This is achieved by working out the failure rates of (almost) every bit of the aircraft and adding them together. (Note: the consequences of the failure are also assessed and those with significant ones are usually analysed in depth. As Mrocktor stated, there are conditions such as no single-point failures etc.) Therefore, the risk due to one system (e.g., flight controls) failing is actually much, much smaller than the overall figure.

However, these are aircraft that are usually designed from scratch with many new and (relatively) untested pieces of kit. In addition, the aircraft will be thrown around in active service, using austere airfields etc.

In the civil world, the hazard analyses should be even more severe (Mrocktor: One in one billion? Wow! That's stringent!).

As Starlionblue mentioned, the risk due to the failure of the flight crew or maintenance is considerably higher than the risk due to the failure of the aircraft. That is why aircraft are becoming increasingly 'stupid-proof'. Unfortunately, nothing is ever foolproof because fools are so inventive...

=======

Anecdote (the truth of which has not yet been verified - if it is true, then RESPECT!):

When they built the Canberra, the ejection seat was put in as an afterthought. However, the ejection sequence would have cut the pilot's lower legs off, since they were underneath the Main Instrument Panel (MIP). So, they designed a system whereby, as part of the ejection sequence, several lines of miniature Detonating Cord (MDC) would obliterate the MIP and then the stick would be pulled into the resulting hole.

Which was great, until a pilot was ferrying one to Australia. On the last leg and still over the Pacific, the pilot was most surprised when his MIP exploded and the stick was pulled into the hole. So, he was left with no stick, no primary instruments, no radio and unsure as to whether the rest of his ejection system was serviceable.

Result? He flew the rest of the way to Australia and landed the jet using just throttles and trim tabs.

Lesson? Both humans and machines can get you into alot of trouble, very quickly. But humans are also very good at getting you down safely.

======

Fear of Flying?

Based on purely personal experience with people who are afraid of flying, there seem to be four main causes of fear of flying:

1. Not knowing what the various bits of the aircraft do and what those funny noises are;
2. Not trusting the aircraft designers to build a 'safe' aircraft;
3. Not trusting the aircrew and ground crew to do their job properly;
4. Not being in control.

I'm not entirely sure what you can do about Points 2-4. However, for Point 1, I agree with Jetlagged. In fact, I have done this with a couple of friends. I took them to the flying club and got the (sympathetic) CFI to walk round the aircraft and explain what each bit does and how it works (Note: I got the CFI to do the explanation, not only because he is an 'expert', but friends seem to believe an independent person more than a friend.). This was then followed by a short flight, to demonstrate how moving the controls affects the aircraft.

I can't say that they've got over their fear, but they are considerably more relaxed now. That said, perhaps someone could explain this phenomenon. Both friends said that they felt far safer in a Cessna 152 than in a 737. Thoughts?



The definition of a 'Pessimist': an Optimist with experience...
User currently offlineMrocktor From United States of America, joined Jan 2005, 1668 posts, RR: 50
Reply 23, posted (8 years 10 months 4 days 6 hours ago) and read 3851 times:

Quoting Bsergonomics (Reply 22):
One in one billion? Wow! That's stringent!

Yes, but that is per failure mode. The total allowance for the aircraft is one in ten million.

Quoting Bsergonomics (Reply 22):
Fear of Flying?

I liked this section of your post very much, matches my experience with folks who are afraid of flying. In general addressing point 1 goes a long way to helping them.

mrocktor


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