Take-off technique - good or not?
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Take-off technique - good or not?
Hi All,
I've been contemplating posting this question for a while. I'm still looking for people's thoughts so here goes.
I'm 32 hours into my PPL, training in a PA-28. I have been taught throughout that the trimmer should be in the neutral position. During the take off roll, however, a little constant back pressure is required to maintain stability and ensure against 'wheelbarrowing'. Is it not worth setting a little negative trim to remove the need for constant back pressure? If so, how many turns - 1/4, 1/2, a full turn?
I've been trying to think of how the aircraft's stability might be affected as the take off roll progresses, and particularly when rotation speed is achieved, but unless I actually put it into practice (which is not something I'm proposing to do anytime soon) I'll not know. Which is where you lot come in
This might seem a trivial issue to many but as a low-houred newbie I want to get into good practices from the start.
Thanks
Tim
I've been contemplating posting this question for a while. I'm still looking for people's thoughts so here goes.
I'm 32 hours into my PPL, training in a PA-28. I have been taught throughout that the trimmer should be in the neutral position. During the take off roll, however, a little constant back pressure is required to maintain stability and ensure against 'wheelbarrowing'. Is it not worth setting a little negative trim to remove the need for constant back pressure? If so, how many turns - 1/4, 1/2, a full turn?
I've been trying to think of how the aircraft's stability might be affected as the take off roll progresses, and particularly when rotation speed is achieved, but unless I actually put it into practice (which is not something I'm proposing to do anytime soon) I'll not know. Which is where you lot come in
This might seem a trivial issue to many but as a low-houred newbie I want to get into good practices from the start.
Thanks
Tim
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The idea behind setting the trim to any specific setting on take-off is that the stick forces during rotation are manageable until you are able to re-trim for the specific CofG that your airplane has, and climb speed you want to obtain.
In large airplanes (talking big iron here) the CofG is calculated in advance and the trim is set accordingly, and I've even seen the same thing in a Gippsland Airvan, for example.
In a light aircraft such as a PA-28, the pitch forces are very manageable throughout the allowed CofG and speed range with the trim in neutral, and there are no markings on the trim wheel, or calculations in the POH, to set the trim wheel anything other than neutral before take-off.
But if you always fly in the same configuration (two up front, full fuel for instance) there's nothing inherently wrong with setting the trim just a bit back. How far back probably requires a bit of experimentation.
But it's really important to realize that once the aircraft gets into a different CofG configuration (solo, less fuel, or four occupants for instance) that that trim setting might be completely wrong and even worse, might be too far aft so that the forces required to obtain the right pitch angle are unmanageable. Although I admit in a PA-28 that's hardly likely. But also, different aircraft might require different trim settings, even with the same W&B configuration.
So if you build your experience with a specific configuration and a specific aircraft, you can wind the trim back a little from neutral, if you find that that's required eventually anyway. But in general, at your stage in training, it's probably better to get used to a bit of stick force at rotation and initial climb-out, and learn to trim as soon as you're airborne.
In large airplanes (talking big iron here) the CofG is calculated in advance and the trim is set accordingly, and I've even seen the same thing in a Gippsland Airvan, for example.
In a light aircraft such as a PA-28, the pitch forces are very manageable throughout the allowed CofG and speed range with the trim in neutral, and there are no markings on the trim wheel, or calculations in the POH, to set the trim wheel anything other than neutral before take-off.
But if you always fly in the same configuration (two up front, full fuel for instance) there's nothing inherently wrong with setting the trim just a bit back. How far back probably requires a bit of experimentation.
But it's really important to realize that once the aircraft gets into a different CofG configuration (solo, less fuel, or four occupants for instance) that that trim setting might be completely wrong and even worse, might be too far aft so that the forces required to obtain the right pitch angle are unmanageable. Although I admit in a PA-28 that's hardly likely. But also, different aircraft might require different trim settings, even with the same W&B configuration.
So if you build your experience with a specific configuration and a specific aircraft, you can wind the trim back a little from neutral, if you find that that's required eventually anyway. But in general, at your stage in training, it's probably better to get used to a bit of stick force at rotation and initial climb-out, and learn to trim as soon as you're airborne.
I remember having a similar conversation with my instructor in the early days.
His explanation was that the "setting to neutral" wasn't so much to actually set it to "neutral" but more to make sure that the trim was not set to max forwards or backwards which would cause a bit of surprise when rotation commenced.
His explanation was that the "setting to neutral" wasn't so much to actually set it to "neutral" but more to make sure that the trim was not set to max forwards or backwards which would cause a bit of surprise when rotation commenced.
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Trim on Take Off
There are many people better qualified to reply than me but my reason for not doing it would be the danger of over trimming 'up' on ground, therefore requiring heavier forward force on climb to keep attitude correct leading to chance of degraded speed if distracted and possible stall!
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I always tend to give a bit of aft trim, the same trim position that keeps the nose-wheel light is the same position for a cruise climb so you transition nicely without having to re-trim.
As has been pointed out, we tend to teach to trim neutral as an exercise to get people to check the trimmer is not completely in the wrong position. When I check a new pilot out you can tell the experienced from the less experienced by the way the trim the aircraft (amongst other tell tales).
As has been pointed out, we tend to teach to trim neutral as an exercise to get people to check the trimmer is not completely in the wrong position. When I check a new pilot out you can tell the experienced from the less experienced by the way the trim the aircraft (amongst other tell tales).
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The idea of taking off with neutral trim is so that as the aeroplane becomes airborn you can feel the pitch forces and verify that they are as expected. You will soon get to know what it should feel like 2-up, 1-up, etc and once you are established in the climb, with the expected amount of back pull on the yolk and it all feels as it should, you can trim off the back pressure for the climb
It's just good practice. You can't accurately trim a light aircraft unless you can feel the forces you trying to trim-out. You can only do that once you are flying. If you 'guess' at a trim setting before you set off, it might be miles out in the air. In practice, once you know an aeroplane, you might give the trimmer a bit of a tweak either side of neutral (I do in the Chippy - a tad more nose-down if I've got someone in the back). But I still re-trim once climbing.
The same does not apply in large transport aeroplanes because they have a much wider CG range and their tailplane setting is therefore pre-calculated for the actual CG.
It's just good practice. You can't accurately trim a light aircraft unless you can feel the forces you trying to trim-out. You can only do that once you are flying. If you 'guess' at a trim setting before you set off, it might be miles out in the air. In practice, once you know an aeroplane, you might give the trimmer a bit of a tweak either side of neutral (I do in the Chippy - a tad more nose-down if I've got someone in the back). But I still re-trim once climbing.
The same does not apply in large transport aeroplanes because they have a much wider CG range and their tailplane setting is therefore pre-calculated for the actual CG.
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Hello Rishy,
I will offer a perspective which I hope you can take in a postive spirit, I am not trying to flame you....
When flying a light aircraft, concentrate less on the "numbers and settings" and more on feeling what the aircraft is trying to tell you about how it is flying. Trust in the engineers and test pilots who have designed and approved the aircraft. They have done all of the testing to assure that even if things are not exactly "centered", even a pilot of modest skill can manage the plane very safely. I certainly agree that if the pitch trim is set way wrong, or the technique is poor, the takeoff can be challenging or unsafe. There is a lot of margin for this built into the design.
At all time when airborne, the plane is telling you how it is flying by the feel. Out of trim condition can be useful in telling you what the plane is trying to do. There are times when I do not precisely trim the plane, preferring to feel the slight changes in control forces.
During takeoff, I do agree that nose up trim is the better default setting, but always prepare for the unexpected. The plane may fly itself off perfectly, or it may need your rapid intervention. The amount of force you could have to apply will be within specified limits (it's a design requirment), but the rate and amount of the application force may require your good piloting skills.
The following very verbose passages are previous writings, which I have pasted in here for you, so as to perhaps offer you a more broad perspective. Hopefully they will remind you of how easy a "regular" flight can be! Don't worry about the numbers and settings so much, concentrate on being prepared, and feeling the plane.
From a recent PPRuNe post I said the following:
PA28's have a stabilator rather than the conventional stabilizer/elevator combination. In "normal" operations this is a completely benign difference. Where it does show up, however, is in very low speed, nose high operations. Here's what happens: Pilot commands more nose up at low speed, takeoff being the most likely occasion. Where the conventional stab/elev combination would have a greater deflection of the elevator, the camber of the horizontal tail as a single flying surface is increased (as well as a change in effective pitch angle). As such, it's capacity to create more "down" lift before stalling, or experiencing a large drag increase is better. (okay purist aerodynamicists, have at me, I've got experience with the result, not the theory). On the other hand, the stabilator, has only the opportunity to change angle of attack to create more "down" lift. Like any flying surface, it will reach an angle of attack, where the Cl max has been passed, and the increase in drag is no longer proportionate to the increase in (down) lift. Yes, I'm sure that Piper gave this a lot of thought and test all those years ago, and their engineers know more than I do about this, but...
I was the right seat check pilot to a junior pilot in an Arrow PA28R-200 many years ago. He rotated prematurely out of a 1000 meter runway. It was a hot day, but we were quite light, and it was a 200HP version of the aircraft, so power was not a problem. The plane was stuck in ground effect, with the pilot holding the controls 'way back. The stall warning was sounding. The aircraft would not accelerate, or climb away. Landing back might have worked, other than the runway end was approaching, and he (well I suppose "we") had allowed the aircraft to drift off the side, so there was not a runway under us any more. I was shocked at the "poor performance" I was experiencing in this aircraft I thought I knew so well. We were nearing the obstacles. We did not wreck the plane simply because I "locked off" and retracted the gear. The resultant reduction in drag was all that was needed to allow the plane to slowly climb away.
Shocked by what had just happened, I went to experiment. I flew a PA28-140, which I believed to be in good condition, off a very long frozen lake (runway and width length no longer a concern). Sure enough, I could get the plane very nose high, with lots of aft control, and it would not accelerate or climb out of ground effect. My only option was to land back (fixed gear). I repeated this configuration enough to satisfy myself that this is a configuration to be avoided in PA28's (and probably Cessna 177's as well) So I do! The thick wing has great lift, and resists stalling well. The stabilator tail is not ideally matched in this attribute.
From an "Incedent" Report, I wrote the following:
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 the 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 – 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 can 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.
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 quick 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 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! 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 authority 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 will offer a perspective which I hope you can take in a postive spirit, I am not trying to flame you....
When flying a light aircraft, concentrate less on the "numbers and settings" and more on feeling what the aircraft is trying to tell you about how it is flying. Trust in the engineers and test pilots who have designed and approved the aircraft. They have done all of the testing to assure that even if things are not exactly "centered", even a pilot of modest skill can manage the plane very safely. I certainly agree that if the pitch trim is set way wrong, or the technique is poor, the takeoff can be challenging or unsafe. There is a lot of margin for this built into the design.
At all time when airborne, the plane is telling you how it is flying by the feel. Out of trim condition can be useful in telling you what the plane is trying to do. There are times when I do not precisely trim the plane, preferring to feel the slight changes in control forces.
During takeoff, I do agree that nose up trim is the better default setting, but always prepare for the unexpected. The plane may fly itself off perfectly, or it may need your rapid intervention. The amount of force you could have to apply will be within specified limits (it's a design requirment), but the rate and amount of the application force may require your good piloting skills.
The following very verbose passages are previous writings, which I have pasted in here for you, so as to perhaps offer you a more broad perspective. Hopefully they will remind you of how easy a "regular" flight can be! Don't worry about the numbers and settings so much, concentrate on being prepared, and feeling the plane.
From a recent PPRuNe post I said the following:
PA28's have a stabilator rather than the conventional stabilizer/elevator combination. In "normal" operations this is a completely benign difference. Where it does show up, however, is in very low speed, nose high operations. Here's what happens: Pilot commands more nose up at low speed, takeoff being the most likely occasion. Where the conventional stab/elev combination would have a greater deflection of the elevator, the camber of the horizontal tail as a single flying surface is increased (as well as a change in effective pitch angle). As such, it's capacity to create more "down" lift before stalling, or experiencing a large drag increase is better. (okay purist aerodynamicists, have at me, I've got experience with the result, not the theory). On the other hand, the stabilator, has only the opportunity to change angle of attack to create more "down" lift. Like any flying surface, it will reach an angle of attack, where the Cl max has been passed, and the increase in drag is no longer proportionate to the increase in (down) lift. Yes, I'm sure that Piper gave this a lot of thought and test all those years ago, and their engineers know more than I do about this, but...
I was the right seat check pilot to a junior pilot in an Arrow PA28R-200 many years ago. He rotated prematurely out of a 1000 meter runway. It was a hot day, but we were quite light, and it was a 200HP version of the aircraft, so power was not a problem. The plane was stuck in ground effect, with the pilot holding the controls 'way back. The stall warning was sounding. The aircraft would not accelerate, or climb away. Landing back might have worked, other than the runway end was approaching, and he (well I suppose "we") had allowed the aircraft to drift off the side, so there was not a runway under us any more. I was shocked at the "poor performance" I was experiencing in this aircraft I thought I knew so well. We were nearing the obstacles. We did not wreck the plane simply because I "locked off" and retracted the gear. The resultant reduction in drag was all that was needed to allow the plane to slowly climb away.
Shocked by what had just happened, I went to experiment. I flew a PA28-140, which I believed to be in good condition, off a very long frozen lake (runway and width length no longer a concern). Sure enough, I could get the plane very nose high, with lots of aft control, and it would not accelerate or climb out of ground effect. My only option was to land back (fixed gear). I repeated this configuration enough to satisfy myself that this is a configuration to be avoided in PA28's (and probably Cessna 177's as well) So I do! The thick wing has great lift, and resists stalling well. The stabilator tail is not ideally matched in this attribute.
From an "Incedent" Report, I wrote the following:
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 the 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 – 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 can 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.
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 quick 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 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! 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 authority 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.
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Some interesting points here, but the best technique I know, that applies to every single aircraft, is the technique described in the POH. Whatever that may be. Vital reading.
Smithy
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I select one turn of nose up trim during the pre-take off checks, which puts me at Vy with hands off the stick. The trim tab position can be checked by looking diectly at it.
Moderator
I select one turn of nose up trim during the pre-take off checks
The trim tab position can be checked by looking diectly at it.
For a number of aircraft, the position of the trim tab cannot be seen from the pilot's seated position, so they'd be ruled out.
For the PA28, which is the subject of the original question, the trim tab angle relative to the stabilator changes with stablilator position relative to the aircraft. Thus, presuming that the trim tab on that aircraft could be seen from the seated pilot's position (which I'm not sure about) the judgement of the trim tab position by looking at it, would be dependant upon the stabilator position, which could be difficult to precisely repeat each time. If the stabilator is pulled all the way back, the trim tab takes quite an angle, which would be very difficult to accurately judge.
For some other aircraft, the trim tab really does not move very much to affect the desired control force change. Thus looking at it is okay, but it could still be away from the desired position, and that not detected until airborne (not that that would really be a problem though, you just trim it more).
For other aircraft, the whole horizontal stabilizer moves. impossible to judge the position by looking at it out the window.
Making reference to the recomended takeoff postion for the trim tab, by looking at the position indicator is a good idea. If the indicator is not correct, or the takeoff position mark not visible, maintenance is required.
Pilot DAR
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flap take off
Alsos remember that the PA28 can have, and I quote 'undesirable pitch moments' when using flap 25 on short field take offs. A other than neutral trim setting could increase that.
Go to the POH and look at the Piper supplement August (or April???)1988 relating to the use of flap 25.
Its only a minor point but worth looking at.
Go to the POH and look at the Piper supplement August (or April???)1988 relating to the use of flap 25.
Its only a minor point but worth looking at.
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Doing a touch and go from a glide approach is always one to watch out for if you forget to re-trim and gives a good lesson in what can happen if you take off with the trim too far back.
You will already have the trim setting quite far back to maintain the glide speed, and when your on the runway and have added full power, the nose tends to rise quite quickly compared to "normal" and at a lower speed, causing you to actually have to push forward on the controls to get the nose back down to a more appropriate attitude to allow you to climb away without stalling, and requiring a rapid nose down re-trim to easy the control forces which can be quite substantial on some types.
Ask your instructor to demonstrate on your next flight.
You will already have the trim setting quite far back to maintain the glide speed, and when your on the runway and have added full power, the nose tends to rise quite quickly compared to "normal" and at a lower speed, causing you to actually have to push forward on the controls to get the nose back down to a more appropriate attitude to allow you to climb away without stalling, and requiring a rapid nose down re-trim to easy the control forces which can be quite substantial on some types.
Ask your instructor to demonstrate on your next flight.
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Pilot DAR, interesting write-up.
I have owned two stabilator aircraft; a PA28-140 and my current Beech C23 (180 hp Sundowner). I have had a couple of surprises on landing the PA28, with full aft stick, and it appears the tail stalled and the nose wheel came down rather suddenly and the landing was more or less a "three pointer" (NOT good on a tricycle!).
I've never had it occur on the Sundowner though. And I have also never experienced the touchy behaviour the Sundowner is supposed to be reputed to have on landing. However I have one of the rare aerobatic models and there is more authority to the stabilator on account if there being a leading edge fillet between the fuselage and the stabilator, which isn't there on the non-aerobatic ones. As it happens the Sundowner is the easiest aircraft I've ever flown for landings (nice quick ailerons in flight as well), as long as you stick very close to the numbers (80 knots with no flaps or 10 deg, 70-75 knots with 25 deg flaps, 68-70 with full flaps), and fly a power-on approach. But I digress!
Still, I agree with you, stalling a stabilator is possible. Didn't in fact Cessna have to modify the stabilator on the 177 with slotted leading edges to avoid this possibility?
Minor quibble, but it isn't a trim tab, it's an anti-servo tab. Without this tab, a stabilator provides no control feel, contrary to a traditional elevator where drag increases as the angle of deflection increases; as you note on the stabilator the whole thing moves and the airfoil section remains constant. The anti-servo tab is meant to add drag and increase control forces in proportion to the position of the stabilator, to give it artificial feel. Of course, it also provides the trimming function but that's not its primary purpose.
My "I learned about flying" story: one year I had my Sundowner fresh out of annual and went flying to do the Transport Canada mandated post-annual test flight. The anti-servo tab attachment bolt to the actuator was not re-installed by the mechanic and I didn't catch it on my pre-flight. I took off and sure enough there is NO control feedback without the tab working. This is actually very dangerous and can lead to inadvertent over-stressing. I figured something was whacky and beat a hasty retreat back for a landing when I discovered my (and the mechanic's) oversight.
Beech
I have owned two stabilator aircraft; a PA28-140 and my current Beech C23 (180 hp Sundowner). I have had a couple of surprises on landing the PA28, with full aft stick, and it appears the tail stalled and the nose wheel came down rather suddenly and the landing was more or less a "three pointer" (NOT good on a tricycle!).
I've never had it occur on the Sundowner though. And I have also never experienced the touchy behaviour the Sundowner is supposed to be reputed to have on landing. However I have one of the rare aerobatic models and there is more authority to the stabilator on account if there being a leading edge fillet between the fuselage and the stabilator, which isn't there on the non-aerobatic ones. As it happens the Sundowner is the easiest aircraft I've ever flown for landings (nice quick ailerons in flight as well), as long as you stick very close to the numbers (80 knots with no flaps or 10 deg, 70-75 knots with 25 deg flaps, 68-70 with full flaps), and fly a power-on approach. But I digress!
Still, I agree with you, stalling a stabilator is possible. Didn't in fact Cessna have to modify the stabilator on the 177 with slotted leading edges to avoid this possibility?
For the PA28, which is the subject of the original question, the trim tab angle relative to the stabilator changes with stablilator position relative to the aircraft.
My "I learned about flying" story: one year I had my Sundowner fresh out of annual and went flying to do the Transport Canada mandated post-annual test flight. The anti-servo tab attachment bolt to the actuator was not re-installed by the mechanic and I didn't catch it on my pre-flight. I took off and sure enough there is NO control feedback without the tab working. This is actually very dangerous and can lead to inadvertent over-stressing. I figured something was whacky and beat a hasty retreat back for a landing when I discovered my (and the mechanic's) oversight.
Beech
Last edited by BeechNut; 8th Jan 2009 at 00:32.
I'm loathe to enter into this, but a true stabilator without the anti-servo tab is unstable. They are designed with the centre of pressure ahead of the hinge line, and the antiservo tab is the only thing which stops the stabilator from diverging to one of the control stops. Quite often the final design does not quite mirror the theory, usually due to the tailplane centre of pressure not agreeing with theory. Theoretically a stabilator requires less surface area to provide the same overall static stability than a fixed (or trimmable) tailplane and elevator, but can have some nasty dynamic characteristics.
Moderator
Yes, quite agreed.
And yes, There was a very early mod to the stabilators on 177's. I have never had any experience with the premod version, which is probably a good thing! I'll talk about my very nasty experience with a premod Cessna 303 tail another day!
Thanks BeechNut and Aeromariner for your informative posts...
Pilot DAR
And yes, There was a very early mod to the stabilators on 177's. I have never had any experience with the premod version, which is probably a good thing! I'll talk about my very nasty experience with a premod Cessna 303 tail another day!
Thanks BeechNut and Aeromariner for your informative posts...
Pilot DAR
I've been flying a 180hp PA28 R a bunch lately, working on my instrument rating. With the trim at the neutral position, it takes some muscle to get the nosewheel off the pavement. I've taken to trimming it a little further aft, lighter forces, I can fly it off with better finesse.
I tow gliders in a PA25. I takeoff with the trim set to about neutral. Helps get it up on the mains easily, and it seeks an attitude which accelerates me to my desired climb speed quickly. I trim it back for the desired climb airspeed once I'm clear of the ground and about to intercept the desired speed.
Trim is all about making your workload less, so you will fly better. Use it as such for all phases of flight, even takeoff.
Neutral is definitely better than the extremes for most aircraft I fly. The C140 likes the trim all the way back for landing. But if you forget to get it forward for takeoff, the tail seems very heavy and hard to get off the ground. It gives the illusion of poor performance, i.e. like there isn't enough airspeed to get the tail up, until it suddenly leaps off the ground. I pulled that goof at a buddy's farm strip, with not very much runway and a fence, trees, power lines at the end. It worked out I had plenty of margin. Probably worthy of the senior moment thread. I've not done that one again though!
-- IFMU
I tow gliders in a PA25. I takeoff with the trim set to about neutral. Helps get it up on the mains easily, and it seeks an attitude which accelerates me to my desired climb speed quickly. I trim it back for the desired climb airspeed once I'm clear of the ground and about to intercept the desired speed.
Trim is all about making your workload less, so you will fly better. Use it as such for all phases of flight, even takeoff.
Neutral is definitely better than the extremes for most aircraft I fly. The C140 likes the trim all the way back for landing. But if you forget to get it forward for takeoff, the tail seems very heavy and hard to get off the ground. It gives the illusion of poor performance, i.e. like there isn't enough airspeed to get the tail up, until it suddenly leaps off the ground. I pulled that goof at a buddy's farm strip, with not very much runway and a fence, trees, power lines at the end. It worked out I had plenty of margin. Probably worthy of the senior moment thread. I've not done that one again though!
-- IFMU
Last edited by IFMU; 8th Jan 2009 at 03:04. Reason: I forgot a y in there somewhere
I figured something was whacky and beat a hasty retreat back for a landing when I discovered my (and the mechanic's) oversight.
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I have about 220 hours on various PA28s.
Must say that I have never felt the need to set the trim other than neutral for take off, which is an imprecise art as others have said.
The stick forces on the PA28 are light, IMHO, more of an issue is the control movement of the yoke require to achieve the correct flare when landing, i.e. up in your chest region
The "feel" of the controls is probably not helped on the PA28 by the rubber seals (or whatever they are called) that sit in the holes throught the various parts of the airframe where the cables run. These seem to me to make the controls feel a little "stiff", rather than heavy, on some examples. Quite different to an Aztec, for instance, with a silky smooth feeling.
One can trim tail heavy to reduce the pull, but in the event of a go around, the consequences of the trim seeting must then be managed.
So I've usually lived with the long pull on landing, even though it feels a little wierd.
Of course, it's a personal matter and I am in no way implying criticism of others with different views, just sharing my experiences.
Must say that I have never felt the need to set the trim other than neutral for take off, which is an imprecise art as others have said.
The stick forces on the PA28 are light, IMHO, more of an issue is the control movement of the yoke require to achieve the correct flare when landing, i.e. up in your chest region
The "feel" of the controls is probably not helped on the PA28 by the rubber seals (or whatever they are called) that sit in the holes throught the various parts of the airframe where the cables run. These seem to me to make the controls feel a little "stiff", rather than heavy, on some examples. Quite different to an Aztec, for instance, with a silky smooth feeling.
One can trim tail heavy to reduce the pull, but in the event of a go around, the consequences of the trim seeting must then be managed.
So I've usually lived with the long pull on landing, even though it feels a little wierd.
Of course, it's a personal matter and I am in no way implying criticism of others with different views, just sharing my experiences.
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Hi All,
Thanks for all the responses, very much appreciated.
It seems some people do in fact set a little aft trim before take-off although I do agree it can be difficult to find the neutral point in the first place. One of our PA28s has a white marker, the other doesn't and I've been taught 6.5 turns forward from fully aft (with two SoB and full tanks).
I'll speak to my instructor next time we're out to get his thoughts too.
Cheers
Tim
Thanks for all the responses, very much appreciated.
It seems some people do in fact set a little aft trim before take-off although I do agree it can be difficult to find the neutral point in the first place. One of our PA28s has a white marker, the other doesn't and I've been taught 6.5 turns forward from fully aft (with two SoB and full tanks).
I'll speak to my instructor next time we're out to get his thoughts too.
Cheers
Tim