So you are flying along and your elevator cable snaps

SixPapaCharlie

SixPapaCharlie

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Anyone have a good diagram illustrating how elevator linkage and trim linkage work together? I have been digging and can't find anything.

Initially I got to wondering if you were flying along and suddenly the elevator linkage broke, if there would be an abrupt change in pitch or if the elevator would be held level in the slip stream and you wouldn't notice until the yoke provided no response.

After a bit of thought I think you would notice nothing at the time of failure as the trim is actually what holds the elevator in its place right?

What I don't understand is how you can have 2 controls attached to the same surface and one does not move the other. I have a stabilator so the trim linkage goes through the elevator to that antiservo tab if I recall the correct vocab.

It seems moving the elevator would cause the trim linkage to tighten and prevent further movement or get a bunch of slack if moved the other direction.

Does anyone have a clear way of explaining or better, illustrating how this works?

Thanks.
 
It varies from plane to plane. In some cases trim might give you some level of controllability in others, you're completely screwed.
 
flyingron said:
It varies from plane to plane. In some cases trim might give you some level of controllability in others, you're completely screwed.
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Interesting.
Not certain why but I was under the impression that GA planes had to have the ability to fly with the trim wheel in the event of elevator linkage failure.

hmmm. It is in the emergency section of my POH but I don't have a clear understanding of how the components work together.
 
sferguson524 said:
ASA 261 comes to mind for me.

http://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261
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I was standing Reactor Operator in #2 plant on the USS Abraham Lincoln when that happened. They rang up our highest flank bell and authorized emergency heat up rates when we were asked to lend assistance. We hauled azz for a couple for an hour or two to get into helo range and help look for survivors, not that there were any...
 
Depends on the aircraft. Some use a completely separate trim adjustment from the elevator control. My AA-5 had this. If the elevator cable broke I could still adjust pitch thru trim. My Glasair trim merely adjusts the push pull tube of the elevator. If that tube breaks or becomes did disconnected (Sean Tucker), I'm screwed. My Velocity has a push pull tube to the elevator as well but uses an electric motor for trim. The tube could break but unless the wires are severed as well, I'll still have control.

Personally I prefer control tubes to cables. More robust and generally more responsive.
 
Velocity173 said:
Depends on the aircraft. Some use a completely separate trim adjustment from the elevator control. My AA-5 had this. If the elevator cable broke I could still adjust pitch thru trim. My Glasair trim merely adjusts the push pull tube of the elevator. If that tube breaks or becomes did disconnected (Sean Tucker), I'm screwed. My Velocity has a push pull tube to the elevator as well but uses an electric motor for trim. The tube could break but unless the wires are severed as well, I'll still have control.

Personally I prefer control tubes to cables. More robust and generally more responsive.
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I have metal rods (I think that is the tube you are referring to) but it also has trim on a steel cable.
 
I can't say for real airplanes, but in the Bonanza, the trim system is completely independent of the intermix of elevator and rudder. I've done the 'no elevator' authority emergency procedure exactly once, and I could get it to stay level in all situations with 10deg of flaps. Trim change with gear extension is -- unusual, you need to spin the wheel very quick to keep it level. In the event of a elevator or rudder failure or both, the Bonanza should be able to get on the ground in a moderately level 'landing' profile using only aileron.
 
On most GA aircraft, the trim is accomplished by a servo tab on the elevator(s) (or, on a Bo, the ruddervators).

If the elevator cable were to part, in most cases, you should be able to fly the plane with trim. I'd be looking for a LOOOOOOOOOONG runway, to which I could make a SLOOOOOOOOOOOOOOW and gradual descent to land.

Edit:

I did not answer your principal question: in most cases, the trim cables should not be affected by the elevator cables; completely separate.
 
The chances of an elevator cable breaking in 90% of the flight attitudes flight are slim. Its most likely to be the "up" cable and right when the most tension is on it like during rotate of takeoff and flare to landing.

If it does break on takeoff I'm trimming to maintain control, contacting ATC declaring emergency then flying to the nearest city with ARFF on the field, trauma center and a long ass runway.


If it breaks on landing something tells me I've probably already got full nose up trim and at least partially flared for touchdown. Think I'm just gonna roll with it since its probably gonna happen so fast there aint much to think about.
 
In the Maintenance Manual, go to 27-30-03. It covers pitch control. Starting on about page 113 it talks about the trim system. It looks like the elevator control and the trim control are separate. As long as the failure mode of the elevator does not bind it in position you should be able to use the trim control for some pitch input.

Jim
 
I take comfort in the Flybaby knowing that the cables are absolutely ridiculously overbuilt for the loads the elevators create. No trim so I'd be pretty screwed and hope to hell I was high enough to jump out.

It just varies type to type. There's absolutely no reason an elevator cable should fail on you if you have a mechanic that is at least barely competent.
 
To answer your question: you would not notice anything IF the airplane was trimmed properly and you had no force on the yoke. (I fly trimmed and with one finger on the yoke)

I have thought about this scenario multiple times and even practiced it in the sim. Yes, it is possible to land with trim only but you might want to practice some ahead of time.

I love my Cherokee design because it has the trim crank on the ceiling and the trim cables are thus separate from all the other cables on the bottom of the fuselage behind the baggage compartment. Should something happen to the stabilator cables, the trim cables are intact.
 
SixPapaCharlie said:
Anyone have a good diagram illustrating how elevator linkage and trim linkage work together? I have been digging and can't find anything.

Initially I got to wondering if you were flying along and suddenly the elevator linkage broke, if there would be an abrupt change in pitch or if the elevator would be held level in the slip stream and you wouldn't notice until the yoke provided no response.

After a bit of thought I think you would notice nothing at the time of failure as the trim is actually what holds the elevator in its place right?

What I don't understand is how you can have 2 controls attached to the same surface and one does not move the other. I have a stabilator so the trim linkage goes through the elevator to that antiservo tab if I recall the correct vocab.

It seems moving the elevator would cause the trim linkage to tighten and prevent further movement or get a bunch of slack if moved the other direction.

Does anyone have a clear way of explaining or better, illustrating how this works?

Thanks.
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If you are in trim, there will be no radical change in normally designed aircraft, they are for the most part inherently stable. You can land on the throttle and trim alone successfully, ask Kim Pardon of the Purple Board to tell you about here experience with this as a student.

As to the individual geometry of how trim functions, there are a few different modes.
 
Barry Schiff covered this in an AOPA article.

He said if one cable breaks, say you can't pitch up, then trim full up and the you have trim working to pitch up and forward pressure to pitch down.

Works just the opposite if you can't pitch down. I am sure this isn't true for all planes though.
 
EdFred said:
Turn in your pilot cert now you jackscrew.
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Hey, I know how to use them but I don't understand the mechanics.
Meaning If I have a pole attached to a door and a rope attached to the same door
Pole = yoke
Rope = trim

If I pull on the pole, the rope will get slack
If I push on the pole, the rope will become tight and prevent the pole from pushing.

My question is mostly a mechanical engineering one.
 
I had a jammed elevator once in a Stinson 108-3 Station Wagon. If you guys don't know how to handle something like that I suggest you spend an hour or so of quality time with one of your friendly local CFIs. I just got back from recurrent, it was a scenario that we had in the sim. In most airplanes, it's not even that big of a deal.
 
Dead Stick said:
I had a jammed elevator once in a Stinson 108-3 Station Wagon. If you guys don't know how to handle something like that I suggest you spend an hour or so of quality time with one of your friendly local CFIs. I just got back from recurrent, it was a scenario that we had in the sim. In most airplanes, it's not even that big of a deal.
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A jammed elevator is opposite from a broken, free flapping elevator. A Jammed elevator you may need to operate the trim backwards.
 
SixPapaCharlie said:
Hey, I know how to use them but I don't understand the mechanics.
Meaning If I have a pole attached to a door and a rope attached to the same door
Pole = yoke
Rope = trim

If I pull on the pole, the rope will get slack
If I push on the pole, the rope will become tight and prevent the pole from pushing.

My question is mostly a mechanical engineering one.
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It helps to look at what's going on when the empennage is opened up at annual. There's pulleys and springs back there to keep tension on your cables.
 
AdamZ said:
I think ASA 261 was s stuck or jammed jackscrew and not a free floating Horizontal stab, but regardless not a situation anyone wants to be in.
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The jackscrew finally galled, and the trim tab was free floating.
 
A participant of this forum had just such an event occur on a taildragger during a training flight, if memory serves. Managed a landing back at her home airport. Cant' recall if the airplane was hurt, but she wasn't.
 
In my POH Emergency Section it says to use the trim for 70KIAS with 10* flaps and adjust power for desired descent rate. It also says to expect to land on the nose if you get the timing wrong in the flare. But it's survivable.

My C-172S has 2 cables for elevator (up/down) and a separate rod for trim.
 
Easy solution to a broken elevator cable: Switch to pedestrian mode and walk towards the light.
 
ClimbnSink said:
Easy solution to a broken elevator cable: Switch to pedestrian mode and walk towards the light.
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That is one potential reaction people will have, however just losing elevator control isn't that big of a deal, so you may end up walking a really long ways to get there.:lol:
 
However losing ailerons is a whole 'nudder game. Not many airplanes come equipped with aileron trim so rudders are all we got.
Practicing an approach (I said "approach", not "landing", for clarity) with just the rudder is a great confidence booster. Of course if one is not confident enough to begin with, a CFI can help.
 
SixPapaCharlie said:
Interesting.
Not certain why but I was under the impression that GA planes had to have the ability to fly with the trim wheel in the event of elevator linkage failure.
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No such requirement. Some aircraft don't even have trim. Some that have it won't result in any useful control.
 
Doesn't answer your question, but, on youtube there is a really good video of a CFI and a student simulating a loss of primary flight controls. The student flies the airplane on a cross country through use of the throttle, rudder, and elevator trim in a cessna 172 I believe. Very interesting. He is not allowed to touch the yoke. He lines up and lands like this also. Good video that relates to your question.


Sent from my iPhone using Tapatalk
 
6PC, I think the part you're missing is that the trim tab "adjusts" the elevator by purely aerodynamic forces. There is no mechanical linkage; there doesn't need to be.

Basically, you change the angle of attack for the elevator, much like flaps or ailerons. There is some discussion of this in the PHAK.
 
There has to be a linkage.
I move my trim wheel and I can see it move when sitting in the hangar.

I am misunderstanding I think.

Edit: No, I know the elevator is deflected the opposite direction that the antisrervo tab is moved and I get that it is aerodynamic force changing the AoA of the elevator

I guess what my question is how there are 2 separate systems controlling the same moving surface and they do not interfere with one another.
 
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Another instructor and I tried landing with rudder and elevator trim only once in a Cessna 152. I had to take over at the last second but it can be done.
 
AdamZ said:
I think ASA 261 was s stuck or jammed jackscrew and not a free floating Horizontal stab, but regardless not a situation anyone wants to be in.
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I recall that the jackscrew seized due to use of the wrong grease by a maintenance induhvidual. Grease is grease...right?
 
SixPapaCharlie said:
There has to be a linkage.
I move my trim wheel and I can see it move when sitting in the hangar.

I am misunderstanding I think.

Edit: No, I know the elevator is deflected the opposite direction that the antisrervo tab is moved and I get that it is aerodynamic force changing the AoA of the elevator

I guess what my question is how there are 2 separate systems controlling the same moving surface and they do not interfere with one another.
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when I prebuyed my PA-28-161, I found the 'up' stabilator cable turnbuckle had broken at the treads and was hanging on be safety wire :eek:!

Needless to say, replacement of the cable was part of the purchase requirements.

So it got me thinking and somewhere on this forum I asked about elevator cable failure. The best reply I got was to trim down while holding back pressure on the yoke to adjust aircraft pitch. The trim will work separately from the stab control. I would not mess with the flaps as my pitch changes a lot when they are deployed.

Anyway, I read somewhere that the anti-servo tab was added,, in part to reduce pitch sensitivity in an attempt to prevent pilot induced oscillations.

I just looked at mine due to my annual inspection. I understand how it works but I can not explain it in words. Perhaps a look at the mechanism with your A&P would help.
 
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The A320 I fly has a THS (trimmable horizontal stabilizer). In the event of a system degradation I can fly the airplane by using the trim wheel for pitch. Of course for landing you would need to select a long runway and there is not going to be much finesse in the landing flare.
 
BigBadLou said:
I love my Cherokee design because it has the trim crank on the ceiling and the trim cables are thus separate from all the other cables on the bottom of the fuselage behind the baggage compartment. Should something happen to the stabilator cables, the trim cables are intact.
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My old Cherokee 140 had enough trim authority that it would be possibly to land it quite well with only trim... that is if the stabilator was not jammed and would still free-float. Additionally the flaps on the Hershey bar wing impart a good nose down movement when applied. All you'd need to do is slow down to flap speed, trim for a bit of nose up and use throttle and flap bar to control pitch.... and a nice long runway too.

My RV-6 also has lots of nose-up trim capability and deploying flaps will pitch the nose down but it would be a much uglier landing without elevator control in the flare. I'd have to find a very looooong runway to try to attempt such a landing. If near my home, I'd head to Sheppard AFB which has 13000 feet and hope for the best.
 
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