Electronics Emergency - Turn off Master?

KS Arrow Pilot

KS Arrow Pilot

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KS Arrow Pilot
A theoretical question for the experts: I was thinking about what to do in an emergency situation when your electronic flight controls fail. I read about a mooney accident where the electronic trim was a suspected cause, and another story about an autopilot in a bonanza that went out of control ( the fuse was pulled and disaster averted).

Let's say you are low to the ground and you use your electric trim to make an adjustment and the trim gets "stuck" and keeps moving without your input. You could spend time searching for the fuse to pull (which would take your eyes off instruments or visual references), or... what would happen if you quickly turned off the master, manually adjusted trim, then took time to find and pull the fuse, then turned the master back on. I've never turned off and on a master switch in flight, so I'm curious what would happen and if this is a logical way out of a serious problem. Thoughts?
 
There are multiple ways to deal with a runaway trim without hitting the master switch. The flight manual supplement for the autpilot or electric trim discusses them. Those are the procedures to follow.

About the only time I'd hit the master in flight would be a suspected electrical fire.
 
If a runaway trim tries to run me into the ground or stall it at low altitude, I'm not going to be piddling with hunting for a fuse I don't instinctively know where it is. With my luck it would be an awkward reach and be a push to turn on only kind of circuit breaker even if it's supposed to be something different.

If I can't stop an electrical problem with normal procedures for that problem, I'll turn the master off. The engine will keep running and all the electrical stuff shuts down right then. THEN there's time to sort out annoying little problems.
 
dmccormack said:
This.

Runaway trim? How about electric trim breaker out?
Click to expand...
Or the AP/Trim Disconnect switch?
Or move the electric trim switch?
Or hold the wheel and overpower the servo or clutch?
The breaker is generally the last thing you try. Master's not even on the list.
 
TMetzinger said:
Or the AP/Trim Disconnect switch?
Or move the electric trim switch?
Or hold the wheel and overpower the servo or clutch?
The breaker is generally the last thing you try. Master's not even on the list.
Click to expand...

:yeahthat:

I have never run into an autopilot who's emergency procedures weren't some variety of exactly that. In fact, my last three airplanes all had memory items along the lines of:
1. Disconnect switch
2. Over power to maintain control
3. Operate trim opposite to runway
4. Pull breaker
5. Continue to over power, land as soon as practical

I don't think the Master would ever cross my mind. What happens if you have the runaway in IMC? Or have to enter IMC to get down? Now you've gone (voluntarily) from a handful that's really no more than an inconvenience, to a full blown emergency.
 
I would think my first instinct would be to grab the wheel and try to overpower the servo too, but in the mooney's case, the pilot was very low to the ground and had to make a decision extremely quickly. Conditions (like flying in IMC, ILS approach, etc) would factor into the decision too.

So is electrical fire the "only" condition that would warrant turning off the Master?
 
Isn't it true that pressing down on the trim switch (as opposed to pushing forward or back) will disconnect the electric trim?

I see that you are a Piper pilot. When I was instructing in PA-28s there were quite a few students who were leery of pushing the throttle all the way forward because doing so brought their hand into close proximity with the (gasp!) master switch! And they knew that because the master had to be on to start the engine, they assumed that shutting off the master would kill the engine, right? So it became incumbent on me to kill the master in flight as a demonstration of the fact that it has nothing to do with ignition. Ten seconds later the master was back on again and life continued...


Bob Gardner
 
TMetzinger said:
Or the AP/Trim Disconnect switch?
Or move the electric trim switch?
Or hold the wheel and overpower the servo or clutch?
The breaker is generally the last thing you try. Master's not even on the list.
Click to expand...

I listed the breaker as the "last resort" before the master...

Also, if you're so hosed you're truly being overcome by the trim, the breaker will kill it dead and good.
 
bobmrg said:
Isn't it true that pressing down on the trim switch (as opposed to pushing forward or back) will disconnect the electric trim?
Click to expand...
On some, but not all.

If you have an autopilot and/or electric trim, you really should know where the c/b's are. What a lot of folks do to help that is put a tie-wrap around the electric trim and autopilot c/b's so they can find them in a hurry by feel. You can also get neat color-coded c/b collars from Sporty's -- for about 10 times the cost of a tie wrap.
 
bobmrg said:
Isn't it true that pressing down on the trim switch (as opposed to pushing forward or back) will disconnect the electric trim?


Bob Gardner
Click to expand...

Depends on the switch. I believe all electric trim controls have two switches built into them that have to both work together. They can be in a side-by-side arrangement or a fore/aft arrangement with an integral button that goes up and down.

The up/down button in this case drives the clutch, and the fore/aft button drives the servomotor. So pushing down on the button should stop a trim runaway unless the switch is broken.

What can vary is the source of the runaway. If it's in the AP computer the trip switch will cause a disconnect and you should be good. But if you had a fault in the switches itself (so that the fault caused the trim motor to run because the circuit was closed as if you'd used the switch), then the breaker may be your only disconnect. But that would require a failure of two switches, not just one.
 
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KS Arrow Pilot said:
I would think my first instinct would be to grab the wheel and try to overpower the servo too, but in the mooney's case, the pilot was very low to the ground and had to make a decision extremely quickly. Conditions (like flying in IMC, ILS approach, etc) would factor into the decision too.

So is electrical fire the "only" condition that would warrant turning off the Master?
Click to expand...

I can't think of anything else off hand. And even then, only if I can't quickly isolate the smoke to some individual component.

While Bob is, of course, right that shutting off the master will do nothing to the operation of the engine or your ability to keep flying, I'd hesitate to "train" it, or even bet on it as a manner of dealing with something (like a/p runaway or uncommanded trim operation) that has many other, less extreme options.

If you have it so ingrained in your mind that you're just going to dump the master and sort it from there, but only if you're in good weather, when the thing actually grabs and it's taking some considerable effort and concentration to hold the wings level, you're going to swat at the master out of pure habit. Now, if you're IMC or in controlled airspace, at the very least you've created a lot of extra paperwork for yourself. That's one of those things that you need to be able to do the same way, every time, without having to sit and think and process your surroundings. Kind of like an engine failure. Why not just find your trim and a/p breakers ahead of time, and keep in the back of your mind where they are? Or even put some of those brightly colored breaker collars on them?

The issue of the Master switch becomes even more pressing as you move into more and more technologically advanced aircraft. If you kill the master on something with AHRS, you may loose your initialization. Now, best case, you have to wait several seconds, or a minute, for it to re-stabilize, and off you go. If you're getting bumped around, your gyros may never re-find themselves. Again, not necessarily fatal, but now you got some 'splainin' to do.
 
Teller1900 said:
I can't think of anything else off hand. And even then, only if I can't quickly isolate the smoke to some individual component.
Click to expand...

Right. The radio master may go off first, rather than kill the master, if you know there's a radio on fire.

Fun part can be if you have a fire in a component (like the Cessna G1000s) that's powered by the standby battery. Killing the master in that case does NOT kill all the components. You have to shut the standby battery off first, then the master.

Moving from a steam airplane to a glass airplane may bring with it habits that are no longer entirely effective.
 
TMetzinger said:
There are multiple ways to deal with a runaway trim without hitting the master switch. The flight manual supplement for the autpilot or electric trim discusses them. Those are the procedures to follow.

About the only time I'd hit the master in flight would be a suspected electrical fire.
Click to expand...

I can think of other times to turn it off. For example in a glass panel plane if above a layer and alt. failure, I would turn it off until i was ready to enter clouds on approach to save as much juice as possible
 
Teller1900 said:
:yeahthat:


I don't think the Master would ever cross my mind. What happens if you have the runaway in IMC? Or have to enter IMC to get down? Now you've gone (voluntarily) from a handful that's really no more than an inconvenience, to a full blown emergency.
Click to expand...

Your gyros, airspeed and altimeter will still work.

I think whoever suggested hitting the master switch meant for it to be a momentary solution. Flip the master off, level the plane, then pull the breaker (or other solution) and flip the master back on.

I'd be inclined to use the above tactic because the master is within reach, and I can practically flip it off in a heartbeat even when blindfolded. To pull the breaker while fighting a runaway trim would be difficult for me. I know where the breakers are (have to reach across pax seat) but to do that while fighting a runaway trim and or autopilot would be difficult.

I know the procedures would be different for a more advanced airplane but for a steam guage spam can I can't see why flipping the master off for 10 or 15 seconds is going to do any harm.

Say you are in IMC, flying down an ILS and your autopilot suddenly does a crazy Ivan. You flip the master off, get established in a climb for the missed approach, pull the autopilot breaker, flip the master back on and advise ATC you are going missed and would like another approach. All of this takes about 20 seconds... no emergency or danger has been created. You'll probably tell ATC you are going missed before they even realize anything is up.
 
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On my electric trim (S-TEC), there are no fewer than THREE WAYS to stop a trim runway:

1. There's an electric trim switch on the panel (and your preflight checklist involves testing it).
2. There's the circuit breaker.
3. You can grab the trim wheel and manually keep it from spinning.

Similarly the autopilot has the disconnect on the yoke, A/P master switch, and the circuit breaker PLUS you *CAN* overpower it.

Again the various disconnects (and the fact that using the trim buttons will kill the a/p if in ALT or VS mode) tests are in the preflight checks you are obliged to do.

In no situation would the MASTER be the first thing I would run for.
 
flyingron said:
On my electric trim (S-TEC), there are no fewer than THREE WAYS to stop a trim runway:

1. There's an electric trim switch on the panel (and your preflight checklist involves testing it).
2. There's the circuit breaker.
3. You can grab the trim wheel and manually keep it from spinning.

Similarly the autopilot has the disconnect on the yoke, A/P master switch, and the circuit breaker PLUS you *CAN* overpower it.

Again the various disconnects (and the fact that using the trim buttons will kill the a/p if in ALT or VS mode) tests are in the preflight checks you are obliged to do.

In no situation would the MASTER be the first thing I would run for.
Click to expand...


I've had an autopilot runaway and it took a surprising amount of strength to overcome. I just flipped it off - had that not worked the next thing I would have gone for would have been the master. I really don't see why having the master off for a few seconds while I pull the breaker is going to cause any sort of trouble.
 
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It all comes down to knowing your aircraft and it's systems.

If I kill the master in my G1000 DA40, I've lost my gyros (ALL OF THEM) untill I hit the emergency battery switch for the standby instruments. So I'd have to hit the emergency switch first, then the master.

Then I'd have to fly the plane on the standby, pull the breaker, start up the master, wait for the ADAHRS to align, and finally get back to normal.

I'd be interested in hearing from anyone who had an uncommanded trim action that DIDN'T respond to the emergency checklist items.

I've had it happen in an M20J Mooney. Something in the static system (water perhaps) made the static instruments and the autopilot think we were in a climb, and it responded with nose down pressure followed by trim rolling nose down. I lost 75 feet (whew) before hitting the autopilot disconnect and hand flying. In the piston single and twin world I just don't see an uncommanded trim incident being that big a deal. I'll let the jet drivers comment on their planes.
 
I've experienced runaway trims on a couple different occasions in two different airplanes (BE76 and PA28). The Duchess was an A/P problem - went rapid full nose down - overpowering it would have been a *****, but it responded to turning off the A/P just fine.

The Warrior was an electric trim issue (non-A/P) related. That was easily solved by turning off the elec trim switch.

Bottom line is (as others have said) know your airplane and its systems.
 
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