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Runaway trim

 
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budyerly(at)msn.com
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PostPosted: Tue Apr 16, 2019 7:05 pm    Post subject: Runaway trim Reply with quote

Sorry I can’t get the list yet in my Windows 10-Outlook.

I have never seen runaway trim. A momentary trim stick and trim failure yes. But a runaway trim button did happen to a friend of mine in a T-33 (Button stuck).

The Europa has sufficient stick authority to fly regardless of trim position. Unless one has limited the stab throw due to a grievous building error, +12to 13 and -4 to 5 gives adequate control.

Building conventions are to install a trim CB where it is easily reached.
There is not a snowballs chance in you know where, you will recognize the problem then get to the trim CB in time to stop a runaway trim. Practice and find out what the control pressures are for various configurations. Today folks install fully self contained power boxes for power distribution, so no trim CB.  I suppose I am guilty of not installing a paddle switch on the stick for a quick trim/autopilot cutout as was military standard back in the 60s for these power boxes.

Trim failure occurs most often with those with the trim button conveniently on the top of the stick… A size 10 boot can break the button and cause it to jamb or be damaged. Flying with a damaged trim button is asking for trouble. Always have a plug at the base of the stick to disconnect the stick. Then use the copilot stick.

I’ve never had the actual trim motor fail, but like all electrical equipment, it can fail.

Wiring gets damaged or disconnected normally. I had an A&P put a drill through a wire bundle one time putting the top on with clecos and hit the trim wire dead on. Found it and fixed it during the build. Normally it is a bad solder joint or poor crimp causes the trim to fail.

If you have never practiced trim failure in your flight tests in any airplane, you are in for a surprise. It can be fatiguing. Plan to land as soon as conditions permit due to fatigue.

Normal trim failure is not runaway, but a trim failure to work (i.e. dead).
Start three mistakes high to practice.
From cruise trimmed up (Fairly nose down trim), try to come in with that trim setting and do a low approach/landing.
From stall or slow flight practice (Nose Up), try that same drill.
Let your imagination run wild. Fly with full nose up or full nose down.

General Rules with full up or down:
Full nose up, cut speed, slow and lower flaps to see if it eases control at approach speed. Do a flapped landing at a speed where you are near neutral stick pressure. With a nose heavy aircraft (forward CG), a no flap landing may be at or near zero stick pressure and be more comfortable. Caution: On go around with full flaps and full up trim, the plane will violently pitch up so use forward stick to keep a steady climb attitude, then bring the flaps up if down to prevent ballooning.

Full nose down, depends on the aircraft. Hold back pressure and slow to do a flapped approach at higher than normal speed near flap limiting speed to ease pressure. You will tire rapidly in some aircraft with full nose down, so flaps at speed will ease your stick pressure enroute to a safe airfield. Eventually you will have to hold some pressure to land.

The old wives tale that the trim can be overridden by stick pressure on a stabilator like the Europa is false. The only possible way for override is if the trim motor mechanism breaks off where it connects to the TS03. Fat chance of that happening.

Others may have found other techniques but practice at altitude and see for yourself what happens with your trim and configuration. Some variations in stab setup causes some planes to have slight differences in trim authority. Same hard over trim situations must be checked for aileron and rudder trim (autopilots too). Older autopilot systems don’t trim so when the autopilot is shut off or fails, surprise!

Those folks who have always controlled pitch with trim rather than setting a pitch attitude then trimming are most susceptible to run away trim out of control situations. I feel it is imperative to know the pitch and power settings and regardless of trim, set the picture, trim it, and then crosscheck it.

My opinion on the 737 Max: Anytime you have an automatic system, those who fail to know their airplane, are at extreme risk. When one is in a new airplane or a modified one, absolute knowledge of what has changed must be committed to memory.
My friends flying at Southwest (at least for another year until forced retirement), had no issues with the Max. Gear Up, Flaps Up, Autopilot On before the plane is stabilized causes automatic stability equipment such as autopilots and the like to misbehave. They simply cut off the auto stabilator incidence stall/trim to reset. This is widely know in the US. Foreign training standards I won’t get into as it is beyond this forum. I was trained to get the plane stabilized, trimmed then hit the autopilot. Fly the plane. Hit the paddle switch (auto system cutout) if the plane misbehaves. Then analyze the situation and take proper action. Know your airplane and practice emergencies.  It is just common sense.
Just my opinion.

Best Regards,
Bud Yerly

Sent from Mail for Windows 10


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