vinwow
Posts: 21
Joined: Thu Mar 16, 2000 9:08 pm

Control Surface Jamming

Thu Oct 24, 2002 7:52 pm

Hi,
Can I get some idea about how the primary control surface jamming is taken care of for aircraft with manual controls?
Are there any means of making such aircarft 'jam-proof'?
I am particularly looking for elevator jamming and its treatment.
Thank you,
vinwow
 
zionstrat
Posts: 225
Joined: Sat Apr 28, 2001 3:26 am

RE: Control Surface Jamming

Thu Oct 24, 2002 10:50 pm

Others will be able to go into much greater detail than I can, but one solution is a control that allows the pilot to manually 'unlink' the pilot and co-pilot's controls theoretically freeing the undamaged cables to do their job.

With hydraulics, many systems sense the opposite pressure applied to both sticks and will allow the stick generating the most pressure to overpower the other stick.

As far as overcoming cable problems, looking at accident history, this appears to be a very unusual problem with well maintained AC. So, in general I would say that the industry sorted this out pretty well in the 100 or so years that cables have been in use. However, there are quite a few reports that list improperly maintained or user modified cables that have failed with predictably nasty results.
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Control Surface Jamming

Thu Oct 24, 2002 11:41 pm

>With hydraulics, many systems sense the opposite pressure applied to both sticks and will allow the stick generating the most pressure to overpower the other stick.<

Huh!! OK, did I miss something??
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Control Surface Jamming

Fri Oct 25, 2002 2:42 am

The Dash8 has separate elevators and cable systems that are normally linked under the cockpit floor. In case of a jam, they can be disconnected. Same idea for the roll control, except that the captains yoke operates the roll spoilers, and the first officer has the ailerons.
 
bragi
Posts: 212
Joined: Sun May 27, 2001 5:17 am

RE: Control Surface Jamming

Fri Oct 25, 2002 2:52 am

If your elevator control fails in some way, you are compelled to use the trim tab on the elevator control surface (which is an independent control system) to control any movement around the lateral axis. It would however, be very difficult to fly a stabilized approach.
If you lose aileron control on a multi-engined aircraft, it is possible to use differential thrust to turn (like the pilots of UAL 232 did).
Fortunately something like this rarely happens.
Muhammad Ali: "Superman don’t need no seat belt." Flight Attendant: "Superman don’t need no airplane, either."
 
zionstrat
Posts: 225
Joined: Sat Apr 28, 2001 3:26 am

RE: Control Surface Jamming

Fri Oct 25, 2002 3:40 am

Not terribly clear, I must admit-I kept it sparse because I read 'manual' to mean cable operated, so I wasn't sure if he was interested and frankly, I can only give an overview, not gritty details-

But larger AC (most airliners) use either cables or digital signals to trigger hydraulic actuators that actually move the control surface. AC in this range also often have split control surfaces- For example, the 747 actually has 2 rudders, one directly above the other. They appear to be one surface most of the time since they are moving together when everything is normal (sidebar, a 747 recently had an un-commanded hard over on the lower rudder and it took the upper rudder, full aileron and differential thrust to make an emergency landing in Alaska- Search this and other forums and you can get more detail).

Unlike the standard cable environment on small AC, this adds the ability to include a monitor loop and to intervene between inputs and outputs- This happens in different ways depending upon the system, but here's two examples-

Airbus uses digital fly by wire inputs that drive the actuators- Since the input is digital, the computers can examine the forces applied to the sticks and make decisions based on what it ‘sees’. As a result, it can add together stick forces when it sees simultaneous input in the same direction from both sticks.

Again, I can’t give the details, but I know that Boeing allows split surfaces to actually move in different directions when the force in opposite directions is great enough- For example the elevator of a 767 is split and one pilot could push down on his side while the other pulled up and the elevator would split in opposite directions (not a good thing, but this is apparently what the pilot and co-pilot were doing in the Egypt Air 767 loss).

Of course the intended use of the split system is referenced in the 747 case above- If one set of control surfaces malfunctions, you still have use of the other side to get home.

Hope this helps-
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Control Surface Jamming

Fri Oct 25, 2002 6:56 am

Interesting note about what would happen if the elevator jammed, and you tried using trim to fly with... You would have to use opposite trim to get the job done, for example, trim nose up to move the nose down and vica-verca... This is because the tab is cabled so that it runs opposite of what it says, and moves the entire elevator the correct way.
 
Beefmoney
Posts: 1065
Joined: Tue Oct 10, 2000 2:16 am

RE: Control Surface Jamming

Fri Oct 25, 2002 7:19 am

I have flown an entire traffic pattern using just rudder and elevator trim to control the Cessna 172. My instructors agree that I should do that every once in a while just in case somthing gets jammed or a wire snaps.
 
Guest

RE: Control Surface Jamming

Fri Oct 25, 2002 7:22 am

FlightSimFreak -
Your statement is incorrect and does not apply to all airplanes -
You are giving false information - which is dangerous with student pilots around who could possibly believe your statement...
(s) Skipper
 
bio15
Posts: 1048
Joined: Fri Mar 23, 2001 8:10 am

RE: Control Surface Jamming

Fri Oct 25, 2002 7:58 am

>>>...Your statement is incorrect and does not apply to all airplanes -


Skipper, that kept me thinking for a while. Are those aircraft in which 'it applies' the ones with control tabs?

-Alfredo
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Control Surface Jamming

Fri Oct 25, 2002 12:53 pm

747skipper... I trust your knowledge in that you have been flying for much longer than I. I only have experiance in a Beech 77 and a 172. This applies to both these aircraft, and I got my answer from a magazine for pilot safety that I read quite often.
 
Guest

RE: Control Surface Jamming

Fri Oct 25, 2002 2:14 pm

The matter is that it is my 13 years old son that read your statement - in which you explain that you could use the trim - YES THAT COULD WORK...
xxx
But then you start to talk about elevator tabs - etc... and explain that it takes an opposite deflection for control... then you get lost in your explanation ... my son asked me " is it true, can we try..." well, the L-21 has a stabilizer which is moved also by cable, but the trim would control the pitch in the proper direction... not the opposite...
xxx
When you or me, or anone make a statement here, which is a forum dedicated to technical or operational data, it is essential to provide correct information, since there is danger that people would use erroneous knowledge that they have acquried here... again for a written test or examination... not important, but in matters of flying techniques - be careful of what you say...
xxx
FlightSimFreak - half of the postings I see are fully correct, the other half, potentially misleading when not complety false... If I make a posting, I will always verify my reference with the concern of accuracy... If aviation is a hobby for you, ok - but many come here in search of valid information...
I did not learn to fly "with magazines", but with approved manuals...
xxx
(s) Skipper
 
avt007
Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: Control Surface Jamming

Sat Oct 26, 2002 2:08 am

B747skipper; A while ago I started a thread about people actually using the info here, and applying it to flying or mtce. Everyone who replied agreed that to do so would be dangerous and irresponsible. In all cases, only the flight manuals or maintenance manuals are to be used. We now return you to our original thread............ Smile
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Control Surface Jamming

Sat Oct 26, 2002 11:50 am

Under the title Jammed Elevators in the August, 2002 issue of AOPA flight training:

"Control-surface failure is an extremely important subject. As with any in-flight emergency, the outcome is directly related to how prepared the pilot is for such an emergency. Emergencies are certainly no time for checking manuals or analyzing the situation for a remedy. Who among us has ever received any training on how to handle jammed, or otherwise errant, primary controls? In all my training right up to CFI no instructor ever brought it up.
Recently I read an article in one of the aviation publications by a flight instructor who had just experienced a fully jammed elevator in the up position while performing a stall with a student in a Cessna 172. In spite of all the physical effort the applied, it stayed jammed. To his credit, the pilot managed to land the aircraft safely - no simple task. The control column had jammed on the thin copper tube that goes between the vacuum pump and the vacuum pressure gauge. A new gauge had been installed a few days earlier.
The very next morning as my student was preflighting my Piper Arrow he called me from the cockpit to tell me that the yoke had jammed in the full-up elevator position! Some work had been done to the avionics, and inspecting under the panel revealed - you guessed it - the yoke stuck on the copper tubing leading to the vacuum gauge. It had evidently been pushed out of the way during the avionics work.
The pilot who saved the Cessna later simulated the jammed elevator condition airborne and realized that trimming nose up would have helped. Think about it - Jeff Pardo, the author of the AOPA Flight Training article did. By trimming nose-up, the 172's trim tab would extend down into the airstream, thereby giving some nose-down pitch.
From now on, all my students will receive training for control problems, as well as a copy of Pardo's article
Vince D'Angelo
Naples, Florida
"



I also got my private pilot license reading from approved manuals, but I also supplemented that "book knowledge" with practical knowledge and other knowledge from people who had "been there" (magazines and asking other pilots) If you fly with only what you learned from manuals, I would not expect you to be a very safe pilot.



I am also interested in knowing what types of aircraft that would not work on... I am trying to further my knowledge, and I cannot think of an aircraft where, if the elevator were completely jammed, opposite trim would not work to counteract it. By the way, are you talking about aircraft with stabilators? Because if you are talking about aircraft with stabilizer, than it would be similar to the 172... Also, what about "Control tabs"...
 
Guest

RE: Control Surface Jamming

Sat Oct 26, 2002 12:59 pm

I assume the C-172 has trim tab for pitch trim, located on the elevator...
In that type of control (trim tab) you would control the pitch as you described.
xxx
However, to control the pitch with a trim on airplanes with variable incidence stabilizer, there, you would not operate the trim "reversed" - I do not know which airplanes do have such type, my L-21 has it... and I do not have a list of aircraft since it is the only light aircraft I fly...
xxx
There are some older airplanes, where pitch trim is in effect a spring force on pitch controls and elevator... that would not work, but it is rarely seen at the present time...
xxx
Agree with you as to going beyond the knowledge of the manual and program of instruction... for as long as that it is done in a manner that is recommended by a reputable organisation, such as AOPA in the USA... What I am concerned about are these "experts" who are working on their third beer at the airfield's bar after having flown their weekly 3 touch and goes... I think they are providing valuable instruction... to barmaids...
xxx
I am concerned because, while I have seen many subjects debated with much accuracy here, I have seen also erroneous statements debated as if they were correct. Example - look at what is being presently discussed about (altimeters) transition altitude/level... There are people that will fly with that acquired wrong concepts they learned... here, and I have to share the same airspace they fly, with my airplane...
xxx
Happy contrails  Smile
(s) Skipper
 
Beefmoney
Posts: 1065
Joined: Tue Oct 10, 2000 2:16 am

RE: Control Surface Jamming

Sun Oct 27, 2002 12:29 am

FlightSimFreak: What you describe about trimming the opposite direction would be effective only if the elevator is jammed and will not move at all. If the elevator is free moving, but does not respond to pilot inputs, then it will not work. The trim tab "flys" the elevator. If the trim tab goes down, the aerodynamic forces move the elevator to a "nose up" position. If the entire elevator is jammed (as in the story above), then you can use your method of trimming because you need to use the trim tab as the elevator, albeit about 1/30th the normal size of the regular elevator.

Just wanted to clear some stuff up.

BTW If I gave out any bad info in this post feel free to bring it up.
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Control Surface Jamming

Sun Oct 27, 2002 3:33 am

That's correct... but if you look, the title of the topic is
'Control Surface Jamming'
 
TWAMD-80
Posts: 962
Joined: Tue Oct 16, 2001 8:25 am

RE: Control Surface Jamming

Fri Nov 01, 2002 6:31 am

I know in the event of a control surface failure in addition to the use of trim you can also use thrust power to control your pitch attitude.

TW
Two A-4's, left ten o'clock level continue left turn!

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