Flight contorls check

[shumway76]

During pre flight in a C172, when you do a flight controls check, with the aircraft stationary is it advisable / permissable to do full rudder delection check? Wouldn't that also cause the nose gear to steer since there is direct linkage to the nose wheel steering?

Otherwise during taxi out, while doing left & right turns that's when you check rudder full travel?

[Pilot DAR]

Good question, but don't worry about it. All 100 series nosewheel Cessnas have springs in the nosewheel steering, to take this motion up. That's why they taxi more sloppily than Piper PA-28/38, which are direct connection pedals/rudder/nosewheel.

In the Cessnas, the nosewheel does not steer in flight, as there is a centering cam which comes into effect when the strut extends all the way. Have a careful look at teh sterring collar, inside the torque links of your 172, and it will start to make sense. Look at teh sterring pushrods, you'll see the crimped end, which contains the spring on each rod.

This is why Cessnas are shimmy damper and torque link condition sensative....

[shumway76]

Thanks for the info. There's a reason why you're with DAR...:)

So whenever the rudder pedals is moved on ground and there is "friction" on the nose gear, the springs will be compressed/expaned, same as when the nose gear is fully extended, the centering cam prevents the nose gear (inner cylinder) from steering.

But if on ground and there is not much "friction" (eg. wet or smooth ground) I'm sure there will be slight movement of the nose gear, and slight scruffing of the nose wheel when aircraft is stationary & rudder pedals are operated full deflection?

[Cows getting bigger]

Before I start the engine. You can hear things going wrong as well as feel them.

[bobstay]

But if on ground and there is not much "friction" (eg. wet or smooth ground) I'm sure there will be slight movement of the nose gear, and slight scruffing of the nose wheel when aircraft is stationary & rudder pedals are operated full deflection?

If it's the tyre you're worried about, I'm sure it gets a lot more "scruffing" every time you land. And if it's wet or smooth ground, it's not likely to harm it anyway.

[Intercepted]

If you convert to a PA28/38 in the future, remember to never check for full rudder travel while you are standing still, especially on soft ground where the nosewheel might dig into the ground. I believe Cessna patented the spring solution wich stopped Piper from using it.

[ExSp33db1rd]

When you do your external you can also move the rudder full deflection left and right - this doesn't prove that it will move when you push the rudder pedals of course, they may have become disconnected, but at the same time move the elevator fully up and down.... and make sure the rudder doesn't bind against it when it is against the (adjustable ) rudder stop, or more to the point make sure that full rudder deflection when recovering from a spin isn't going to inhibit what you want to do with the elevator, or v-v.

( if the rudder travel can be adjusted, it can be adjusted wrong. The most dangerous aeroplane to fly is one that has just come out of maintenance. )

Murphy is always with us.

[Pilot DAR]

While we're chatting about flight control checks, I'll repost this, perhaps for a new audience here. It's pitch, not yaw, but contains some important "lessons learned" none the less............


So you’re going to do a maintenance test flight….

Part One - Which way is up?

A maintenance test flight was required, prior to the Transport Canada approved test flight for the evaluation of an external installation to the aircraft. I had flown this aircraft months earlier, for a very similar mod evaluation purpose, including spins and dives to 1.1 of Vne. Those flights were fine. It had not flown since, while it underwent inspection for a commercial C of A. This was the maintenance test flight following that inspection.

The aircraft is a Cessna U206F, with a Robertson STOL kit, and additional external equipment mounted. Following a review of the documents for the maintenance, a thorough preflight inspection, and normal start, I taxied out. Just before taking off, a final check of control freedom and direction, including the elevator trim tab – I have just read too many stories about flight control problems on test flights… Everything looked as it should from the pilot’s seat.

The aircraft was light weight at takeoff, and promptly leapt off the runway… Then immediately leapt more. A swift and large nose down control input seemed to help, but still it was heading for space! I confirmed that the pitch trim was set for takeoff (and it had been) then I rolled it all the way nose down. That helped a bit, but not really much.

By this point, landing back on the remaining runway was no longer an option. A circuit now lay ahead of me, which was going to be a muscle builder! I could lie on my back and hold my 30 pound daughter at arms length over me for a few minutes at a time. This flying was a lot more demanding than that! Flying with one hand so as retract flaps (very carefully) and adjust power, was manageable for only a few moments at a time. In downwind, I found that full flaps created the least uncomfortable configuration, probably simply because slower speed, less control force. I did not dare fly too slowly, being quite uncertain about how the aircraft would handle if stalled this way. During downwind, I was thinking about how I might jam my knee into the control wheel so as to relieve my tired arms – there really was no practical way..

Flaring for landing was an exercise in how to appropriately reduce the massive downforce I was applying to the controls. It worked. I taxied in, alternating thoughts of shock, and the old joke: I just flew in from – here! – and are my arms tired!” I also reminded myself how lucky I was there were no seats, occupants, or flight test ballast in the back for this first flight….

Knowing that I had had a serious pitch control problem, I asked for a check of the elevator and trim tab travels. The elevator was as it should be. The trim tab offered 3 degrees, where I would have expected to find five degrees according to the type certificate data sheet. I asked that the maximum travel in this direction be provided. I got 7 degrees. There was still a lack of certainty as to what the problem was, because I could not see how such a small travel limit error could produce such a dramatic effect. I invited the mechanic to join me in my next test flight. He reluctantly agreed, knowing that if he would not ride in it, why would I fly it. This time I was well prepared to abandon the takeoff, if things were not right. The takeoff was better, but the pitch control problem was still there. what had been a 40-50 pound push, was now 15-20. The mechanic now had no doubt that something had been very wrong on the first flight, as was still wrong now. I landed back.

After a rather puzzling review of the maintenance accomplished since my flight a few months earlier , the answer was found. The maintenance personnel had put a little too much thought into what they were doing…. The required maintenance had included the required overhaul of the trim actuator, thus it, and all the chain and cable, had been removed, and reinstalled. During the re-rigging, the technician had read the travel requirements for the tab in the maintenance manual. Instead of setting the tab for an up travel limit of 25 degrees, he set the travel to a “nose up” (tab down) limit of 25 degrees. This left the travel limit in the other direction of only 5 degrees, which I hereby attest is not anywhere close to the requirement!

By trying to “figure out” what the system required, the actual instruction was not followed as written. An unsafe condition was the result. The safety system further broke down, when the second signatory for the work accomplished did not detect the error. This was also a maintenance failing designed in by Cessna, as it was possible to mis-rig the system in the first place, and the manual did not give any warning to check for the mis-rigging.

Part Two – Is there enough?

With the trim error corrected, and many sets of eyes and minds making sure everything was just as it should be, I had the aircraft loaded so as to be at maximum gross weight, at the aft C of G limit. Off I went again. The elevator trim worked, well, though I was not completely sure of the indicator position. I was, though, satisfied that the aircraft was now conforming to its design.

I climbed the aircraft very high as I had done before, to do stalls and spins as required by the design approval test flight plan. The power off stall was very normal. While setting up for the maximum continuous power, full flap stall, things started to go wrong again…

With the power set, and the flaps selected to full, and passing 20 degrees deflection, I reached the forward control wheel travel stop. The nose was rising quite quickly now, with no ability to stop it, as the flaps continued to extend. The trim was set to full nose down, but that was really not a factor anymore. I had no more control! The only resolution I could think of was to retract the flaps as quickly as possible. Reducing power did not seem a good idea at such a nose high attitude. The stall warning was now screaming, and who knows what kind of stall recovery I would have if I could not lower the nose!

The flaps retracted back through 10 degrees just as the plane began to mush rather badly, settling downward quite nose high. I got it all sorted out, and resumed normal flight. Being up high, I decided to explore this situation, to try to fully understand it. Obviously another discussion with the maintenance crew would be in order, I’d better have something to tell them which was helpful…

I set up again for the power on stall, this time feeding in flap a little at a time. Sure enough, at 20 degrees flap the control wheel was at the forward stop again. I found that with lots of muscle, and the elevator trim set at full nose up (which caused a little more effectiveness of the elevator, because of the downward tab), I could get 30 degrees of flap down, and control aircraft pitch with slight flap setting changes. The strength required to fly this way prevented doing it for very long.

So I took the plane back to the mechanics, and reported that it still did not fly right. After a review of the loading for the flight, an error was found in the basic weight and balance. I had been 150 pounds too light! And 1 inch aft of the aft limit. Could this combination result in these poor flying characteristics? I thought not, but we reloaded, and I went again. Nope, it flew the same way. Back I went.

After a complete re-inspection of control travels, and the system, it was found that a previously undetected broken bearing in the elevator bellcrank was affecting the elevator travel. The cable tension made this not immediately apparent during a walk around control check. I would have thought, that such a defect would have been found during the recently completed commercial annual inspection. Oops!

It was also found that the horizontal stabilizer (which is not adjustable relative to the airframe) was more than one degree beyond its specified angle of incidence – but in the direction which would improve pushover control! There is no adjustment for the H stab on the tailboom. The bellcrank was removed, and the bearing replaced, with the expectation that the elevator travel would now be correct. It is noteworthy that this model year of the C206 specified a bushing, not a bearing, so it had been changed at some previous time, but no technical record entry could be found to describe this work.

I test flew again…

Part Three – Please sir, may I have more?

With the bellcrank reinstalled, and the elevator re-rigged, the problem was now worse! The elevator deflection with the maximum possible travel, and farthest stop setting, was even less than it had previously been. The broken bearing had been improving the elevator’s range of travel! I landed back.

All of the other elevator control system parts were checked and found to be correct applicability, and in good condition. There is no Cessna design provision to adjust out this problem. The only remaining possibility was a modification to the elevator control stops. It was possible to completely remove the Cessna installed elevator control stop block, leaving only the bolt as the stop. This did allow just enough elevator travel that the aircraft could be safely flown through all of the phases of flight. I tested very thoroughly, and found the aircraft to be acceptable.

It is sobering to realize that this aircraft had flown for years following the repair that had the horizontal stabilizer incorrectly installed, and the wrong parts put into the elevator system. In this configuration, had a pilot used full flap and high power at the same time, with an aft C of G, they would have run out of elevator effectivness instantly. Close to the ground, at could have been fatal. It is total luck that this never happened. In such a case, it is unlikely that the accident investigators could ever have determined that a bent airframe had not been correctly repaired years earlier. The cause might have been reported as “pilot failed to maintain control”, rather than the very different “pilot could not maintain control, due to airframe defect”. I also reminded myself that my month’s earlier test flight, which should have caught this defect, did not. I have to be more thorough, even though what I might find is not what I was up looking for.

I contacted a Transportation Safety Board friend of mine, to informally report that this had happened to me. After explaining the whole thing, he said “that happened to you too?”. Apparently this trim misrigging had also happened to Transport Canada’s own C 206, with the same scary, muscle building, yet accident free outcome.

It the subsequent times, I told everyone I met, who were associated with C 206’s about this, just as a word to the wise. Some time later, a fellow called me, and asked if I would come and fly his 206, it was doing the same thing ( though with less force), which he had remembered my describing. I went to the airport, and asked that the control and trim travel limits be measured in my presence. It was measured, and the trim was wrong (though not backward). I had the mechanics put it right, flew the plane, and all was fine!

[Desert185]

In the sim with uncontrollable, full runaway up trim, we used to teach rolling into at least a 30 degree bank and slowing with flaps to relieve back pressure until the trim could be reset. You can maintain that flight condition until fuel exhaustion and the forward pressure will be comparatively minimal during the wings-level portion of the approach. It's called involuntary trimming during a steep turn. ;)

[Gertrude the Wombat]

During pre flight in a C172, when you do a flight controls check, with the aircraft stationary is it advisable / permissable to do full rudder delection check?
Whether it's "essential", and you get a bollocking for not doing it, or "forbidden", and you get a bollocking for doing it, would appear to depend on the instructor. So, just remember what each instructor wants to see.

[Pilot DAR]

Thanks Desert, that's a trick I had not thought of, but I sure see the wisdom. I was so busy just to keep the thing flying straight and level, I did not think to get it into a turn! (and I used to fly jumpers, I should have thought of that!) I sure don't expect to get caught by this again, but I'll keep the trick up my sleeve. Thoughtful posts like yours remind me why I enjoy PPRuNe!

[frontlefthamster]

Knowing that I had had a serious pitch control problem, I asked for a check of the elevator and trim tab travels. The elevator was as it should be. The trim tab offered 3 degrees, where I would have expected to find five degrees according to the type certificate data sheet. I asked that the maximum travel in this direction be provided. I got 7 degrees. There was still a lack of certainty as to what the problem was, because I could not see how such a small travel limit error could produce such a dramatic effect. I invited the mechanic to join me in my next test flight.

Pilot DAR, may I ask a couple of questions please? First, I presume you wrote the defect up on arrival, so how did maintenance clear the defect for flight or was it done on conditions, and second, given the lack of certainty, why did you carry out a second flight? I ask because I have been in a similar situation (though on a much larger twin jet aircraft) though the whole thing played out very differently.

By the way, the roll for loss of control in pitch advice is excellent and should be more widely known; it's especially helpful in case of runaway trim on departure. All of us who do testing should know the basics back to front. Neil Williams' excellent write-up of his horrid flight with a failed spar is a good ILAFFT treatise and should be compulsory for all: Zlin wing Structural Failure Report - Neil Williams (http://www.aerobatics.org.uk/repeats/zlin_wing_failure.htm)

[ExSp33db1rd]

Canada again, RCAF Station MacDonald, circa 1955.

Friend over the hedge in a T-33, about to start the landing flare etc. ELECTRIC elevator trim 'ran away' full nose up, so he flung it into a 90 deg ( approx) left turn and applied power again whilst he fumbled with circuit breakers and alternate trim system with his spare hand, to get the beast under some sort of control again.

I'd have been dead.

[Pilot DAR]

I presume you wrote the defect up on arrival, so how did maintenance clear the defect for flight or was it done on conditions, and second, given the lack of certainty, why did you carry out a second flight? I ask because I have been in a similar situation (though on a much larger twin jet aircraft) though the whole thing played out very differently.

Well... the aircraft was on an experimental flight permit, so a C of A was not in force. Though I did not formally write up the snag, I can assure you that I had the 5 mechanics who had a serious interest in the flight test standing there when I returned. They knew that if I flew a circuit that small and short time, something was wrong. The second flight was carried out with a much better thought out abort criteria and procedure (my common practice - now...). As the aircraft was much improved, though still non compliant. I wanted the mechanic I took with me to get the idea of the problem. In truth, for the second flight, no one had yet figured out that the trim stop limits were actually reversed. All they knew was they had already given me more than they thought would be necessary, so I kinda had to prove to them it was not right yet.

I did not sign of the "fit for flight" maintenance test flight release for seven subsequent flights, 'till they finally got it flying properly. Without that signature, the aircraft could not be test flown for the modification test flight I was originally supposed to be doing.

This is certainly not the only time I have done a maintenance test flight with an aircraft with a hidden defect, it's just the worst example. A Cessna 185 floatplane flight test I did was equally embarrassing for the flight control rigging (rudder in that case) which was done wrong (springs missing), just less dramatic. There were others. It's surprising how much error a mechanic can "maintain" into an aircraft, and a careful pilot can still fail to detect before flight! Maintenance test pilots beware!

I take it that your large twin jet had a less good outcome. Any thing we can learn from the whole story?