PITR

(P)ilot (I)nduced (T)rim (R)unaway - for those that don't create acronyms in their sleep, like I seem to do....

I had an interesting experience - totally my own fault) on my recent trip up to Oshkosh - thought that sharing it might help others to recognize/understand what can happen. I was cruising along, VFR above a haze layer, autopilot on, no bumps - everything normal, just waiting for the mileage to the fuel stop to click down. I had pulled out my bound chart book and was looking at some interesting feature to pass the time when I suddenly felt a significant pitch up, looked up, and saw clouds - the airplane appeared to be trying to loop!

My first reaction was forward pressure on the stick, and that quite quickly and successfully arrested the motion, so nothing bad happened - I probably gained 300 feet total. I started trimming nose down, and glanced at the indicator - sure enough, it was significantly nose up. My first thought was a malfunction, but that lasted only a second before I recognized what had happened. I had obviously hit the coolie hat with my map book. I do not have a trim sensing altitude hold unit, so it held altitude as long as it could, and then suddenly disconnected when the forces became to much for it. That accounted for the "sudden" nature of the pitch up. According tot he G-meter, I never got more than 2 G's (and that might have been leftover from a turn on climb out), so it really wasn't much of an event - but could certainly surprise someone if they'd never heard of it.

I'm not going to over-react and include a trim disable switch for when I'm reading maps....just going to be more careful now that I had it happen. I guess the most significant "new" information was how the autopilot let it get significantly out of trim before releasing. Watch for it, don't panic - and don't' bump your trim hat with your maps!

Ya' learn something new every day....or you're not trying hard enough!

Paul

Another PITR story from my RV-6A first flight report

I was cleared for take-off, then told position and hold. So my pulse shot up, then back down.

Cleared for take-off, I eased the throttle up, watched engine gages, kept her on the center-line, then up she went, with a heavy climb and heavy left wing. Strong stick force was required to keep her level. Uhhoo, whats wrong?

"Hold what you got", I said to myself. She was in a good climb and I had control of her. As I turned right cross wind, at 1500ft agl, I looked at the trim's and they were pegged in the full up and full left position. "****, must be a short. I know they were centered in run-up." So I put them back into center and she leveled off nicely. Feeling comfortable now, I look for the chase plane, which I had already climbed past at 3500ft agl. As I looked for the plane, again I feel some strong force on the stick, and again the trims are pegged. "****, must be a short. Do I land? How am I going to trace all those trim wires? Thats gonna take a week of pulling stuff off." Little things going through my mind as I flew along to 4000ft AGL. Fighting this trim issue for 20 minutes. I noticed if I stared at the trim indicators when they were centered, they did not move. How can that be? How do they know I am looking at them? If I stared at them for 30 sec., they would not move. Take my eyes off them for 10 seconds, they were pegged again. What gremlin is messin with my trims?


Then, I caught them moving on their own, with my left thumb, without input from my brain, was resting on the hair trigger trim buttons on the stick and running them to their end stops. "Oh ****, ITS ME!! Get you thumb off the buttons *******!"

Darn...

There go the naps...
One feature I like on the TT DFII VS is the trim sensing, it doesn't adjust trim for your but flashes a trim indicator at you telling you it's time to trim and which way it needs to go. You just hit the trim until in stop annunciating and you're good to go

Hey Paul!!! We saw you land at Oshkosh. We were down by the seaplane base pickup point in the south 40 and thought I saw your plane on short final on 36. Recognized the tail artwork. Checked the registrations later and sure enough it was you.

Roberta

Electric vs. Manual?

Can we start another endless debate about electric vs. manual trim because of this? Nah. Can we start one about having the trim button on the stick? Nah. Those would be rubbing it in. Nonetheless, I considered electric and I considered having it on the stick and stuff like this is part of the reason I went manual. I even have manual aileron trim, but I've never used it. No rudder tabs either. It's a QB; I can't take the credit.

Did I mention that for a while my TT AP would pitch up sharply when I transmitted on the SL-30? I guess nobody's immune!

I have my electric trim switch on the arm rest on my left side. I've had PITR also <g>. Buddy Scott who was my pax still gives me grief over it.

robertahegy said:

Hey Paul!!! We saw you land at Oshkosh. We were down by the seaplane base pickup point in the south 40 and thought I saw your plane on short final on 36. Recognized the tail artwork. Checked the registrations later and sure enough it was you.

Roberta

Click to expand...

Sorry I missed you Roberta - entirely my fault for having such limited time there - 24 hours and I was a pumpkin, on my way back out!

another way to excite the trim?

Paul and others, can you induce a runaway trim by simply pushing or pulling on the stick? For example, in altitude hold, if you push on the stick, some autopilots will start trimming up. If you keep holding altitude by pushing more, the trim keeps running. Regards, John.

John C said:

Paul and others, can you induce a runaway trim by simply pushing or pulling on the stick? For example, in altitude hold, if you push on the stick, some autopilots will start trimming up. If you keep holding altitude by pushing more, the trim keeps running. Regards, John.

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On my simple system you can't John - I don't have a trim-sensing servo - I can induce an Autopilot disconnect, but the autopilot and Trim have no connectivity (Altrak and R/C Allen Trim). Some of the more complex autopilots might...

Paul

I had a PIAPD, Pilot Induced Auto Pilot Disconnect, flying the Tiger back to Savannah from NC. I was looking at the chart and didn't realize I hit the Autopilot disconnect button. I looked up and I was diving towards the ground. The only thing I have on my RV-10 stick is the push to talk button. I figure the worse I could do was call somebody. I had to idiot proof the stick.

I managed to trigger full up trim while flying a loop in an 8A during the latter stages of the test period. Scared myself silly when I needed a stick push for the last quarter and into level flight. I was sure something had broken. Pulled power, turned for the airport...and then noticed the trim indicator.

Maybe this on the panel would be better. Not as fun probably.

http://www.aircraftspruce.com/catalog/elpages/racswitches1.php

Rivethead said:

Maybe this on the panel would be better. Not as fun probably.

http://www.aircraftspruce.com/catalog/elpages/racswitches1.php

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That is exactly what I put in!

Could we get the annuncator wired to the seat.

Walt said:

There go the naps...
One feature I like on the TT DFII VS is the trim sensing, it doesn't adjust trim for your but flashes a trim indicator at you telling you it's time to trim and which way it needs to go. You just hit the trim until in stop annunciating and you're good to go

Click to expand...

That way Paul and Kahuna would get a little jolt, I mean reminder, to indicate to look at there trim indicators.

Kent

disconnect

Way off topic but reminds one of the disastrous DC-10 crash in the everglades years ago. The whole crew was trying to figure out a nose gear malfunction (nose gear down light inop?), looking down into the lower nose area and the autopilot altitude hold was apparently knocked off. The plane descended into the muck with great loss of life. If you were distracted enough and the descent gradual, it could be real threatening. Bill

Paul
I had exactly the same thing happen to me while on a long cross country flight with my wife. I had the autopilot on and the map spread out on my lap, over the stick. Out comes the airport directory book and it got held up by the map, which was being held up by the elevator trim on the stick. All of a sudden we had a huge pitch up. my first reaction was to grab the stick and break the climb. This caused a few things to float around the cabin and my head to contact the canopy. It was not a good start to a long trip as I was at first not sure what had happened. Since then I have been thinking about a little trim disable switch beside the button that ativates the altitude hold. A pitch up was scary, a pitch down of the same magnitude would have put us in a high speed split S in the blink of an eye. This is something that we all need to put a bit of thought into. Perhaps the people at trutrak could make an intergal disconect on the altitude button for the trim ?

Recovering from unusual nose-high attitude.

This is one to practice so that it will become second nature in real life. When you suddenly realize that you are in an unusual nose-high attitude, your first reaction might be to just shove the stick forward. This can create a dangerous negative "G" condition, especially in the non-aerobatic types like the -9's and -10's. The correct response is to apply power if your airspeed is decaying, then roll the plane into a steep bank (80-90 degrees) and just let the nose fall to the horizon. Then simply roll level. No negative "G's" imposed on the airframe. This is how it's done by the heavy iron and bizjet types.

Ron
I do not have any control time in heavy iron or bizjets but I have lots of RV time and about 1000 hours in rockets and I would not like to even contemplate what position your recovery technique would put me in. Knowing what I know now I would not push forward as aggressively as I did but a reduction of power is what I needed not an increase.This was not a gradual increase in nose up attitude, it was an instant pull up created when the autopilot could not hold a trim demand for nose up. I was well on my way to doing a very tight loop. When the trim is calling for nose up it is a hard push to get the stick forward before I could even begin to bring the trim back to level flight. Having gone through this once I would be much better mentally prepared as I would at least know what was going on!
Your comments about awarness and practice are spot on but I am not sure the techniques of heavier airplanes apply here. A discussion from others would be most welcome on this topic.

I don't want folks to over-react to this, of course - just one of those little lessons learned. Not really a big deal in the scheme of things - you can easily control the airplane with a full out-of-trim condition, and believe me, when the autopilot disconnected, there was no delay in the seat-of-the-pants indication that it was pitching up!

Paul

Tom Martin said:

Ron
I do not have any control time in heavy iron or bizjets but I have lots of RV time and about 1000 hours in rockets and I would not like to even contemplate what position your recovery technique would put me in. Knowing what I know now I would not push forward as aggressively as I did but a reduction of power is what I needed not an increase.This was not a gradual increase in nose up attitude, it was an instant pull up created when the autopilot could not hold a trim demand for nose up. I was well on my way to doing a very tight loop. When the trim is calling for nose up it is a hard push to get the stick forward before I could even begin to bring the trim back to level flight. Having gone through this once I would be much better mentally prepared as I would at least know what was going on!
Your comments about awareness and practice are spot on but I am not sure the techniques of heavier airplanes apply here. A discussion from others would be most welcome on this topic.

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Shoving the stick forward in any aircraft, whether a J-3 or a B747 will create a negative G condition. Weight has no bearing on this. The technique I described is the way to recover from an unusual nose-high attitude, and is practiced in every type rating and recurrent training I've ever gone through at Flight Safety. It is simply unloading the airframe and not subjecting it to negative G loads. This is all practiced "on instruments", which of course makes up the majority of training at FS.
This type of training allows you to evaluate your unusual attitude situation, and develop a response without having "fast hands" in the cockpit. You can still practice this in your aircraft using proper safety techniques.

On another matter, I don't believe I've ever seen an installation of a trim switch where upon activating it, it did not disconnect the autopilot.

I will defer to your flight safety experience and after having reread your initial post I think I understand what you are suggesting. I will try this the next time I am flying. The basic trutrak altitude hold systems
are not connected to the trim system. It would be a very good idea if it were though but that in itself would require some sort of warning to alert the pilot that the autopilot had been disconnected.

Tom Martin said:

I will defer to your flight safety experience and after having reread your initial post I think I understand what you are suggesting. I will try this the next time I am flying. The basic trutrak altitude hold systems
are not connected to the trim system. It would be a very good idea if it were though but that in itself would require some sort of warning to alert the pilot that the autopilot had been disconnected.

Click to expand...

Most larger planes have some kind of an aural warning tone to let you know that the A/P has been disconnected, and some alt.hold systems have a warning if the altitude goes astray from set by 300 feet or more. An avionics shop should be able to help here. I can't see any reason to be trimming against the A/P.

<<It is simply unloading the airframe and not subjecting it to negative G loads.>>

Ron, I'm hard-pressed to understand the concern with negative G in the context of an RV. No problem understanding the concern with heavy iron; in addition to airframe load constraints there are obvious concerns with passenger and flight attendent safety (floating people, laptops, drink carts, etc).

No practical airframe constraint with an RV because the natural response for almost everybody will be to push to zero G and not much beyond. Non-acro pilots find negative G unnatural/uncomfortable; they correct stick to eliminate it without thought.

In no way am I saying the FS-trained method isn't better, but it does require training to develop the preset response. Sadly most pilots don't even practice stalls or steep turns. For them the roll method will be a matter of inducing a new unusual attitude to correct the first one.

If you've aready gone vertical or beyond, a straight push is clearly a poor way out. However, "roll and let the nose drop" is likewise no longer a simple process. Honest curiousity; does FS training include a pitchup that has gotten that far? How much pitch rate can you generate with an airliner anyway?

DanH said:

If you've aready gone vertical or beyond, a straight push is clearly a poor way out. However, "roll and let the nose drop" is likewise no longer a simple process. Honest curiousity; does FS training include a pitchup that has gotten that far? How much pitch rate can you generate with an airliner anyway?

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Hmmmm.... How much pitch rate can an airliner generate? Been on one lately?
If you could completely reshape the wing on your RV like the airliner can you could probably generate and sustain those takeoff pitch angles too! Airliners have been rolled and looped, so yes, they can be found in any attitude!
Anyway, in the simulators, unusual attitudes are taught by the instructor having you (the pilot) putting your head down and closing your eyes, while he puts the sim in an unusual attitude. When he has it where he wants it, he will tell you to look up and recover. You can be 40-50 degrees nose down, lots of bank angle, full power on, and the airspeed screaming over Vne.
Plus, it will be at "night", IFR, with no outside references! He may also fail your primary flight display (gyro), he, he, during this maneuver so while you may think you are banked right, it may be the exact opposite.
You need to evaluate the situation by cross-referencing your gyro with the co-pilots gyro and the stand-by gyro. Pick the two that match and fly those. Since you are in a jet, if you're overspeeding you can just yank back the thrust levers, deploy speed brakes and then work on rolling level and pulling pitch to regain level flight.

The other scenario will be nose-high (50-60 degrees or more), power usually low or cruise, and probably a small degree of bank angle. Your airspeed will be rapidly decaying. After cross-referencing instruments to be sure of your situation, power on immediately because it takes some spool-up time, plus you have inertia working against you, continue banking in the direction of the already low wing until you have an 80-90 degree bank angle and let the nose fall to the horizon. As it is nearing the horizon, start gently rolling wings level.
Reduce power as needed when your airspeed has returned to a safe number.
Recovery complete! This is a very smooth, comfortable maneuver that won't stress the airframe.

<<those takeoff pitch angles>>

Sorry, I snuck in that last question. Pitch rate, not angle. For example, in checking gyro loads on a propshaft you might assume at least 6.28 radians/sec for an aerobatic installation, meaning 360 degrees per second pitch or yaw. Obviously airliners don't do that...we hope.

Is pitch rate or G limited with safety devices or is that just for AOA? I suspect jet-class velocity makes G the limiting factor in pitch rate, not control authority. The roll-and-drop proceedure would work at any speed without bumping limits, so it makes a lot of sense.

<<Anyway, in the simulators,..>>

Very cool, thanks for the explanation. I've heard the top line simulators are very realistic. Surely some joker has programmed the sound of puking passengers and swearing flight attendants? <g>

It's those gyro loads on propshafts that make me want to shy away from prop extensions, PSRU's in general, and any other drive that wasn't designed for the pitch rates that these planes can develop.
The most realistic effects I have experienced in simulators is sound of the instructor's groans, and the laughter coming from the right seat guy when I screw-up an approach.