Nope, we are not talking about George Clooney and his band of runaway felons in the movie “O Brother, Where Art Thou?” In that movie the felons wore striped clothing and were easy to recognize. They ran across fields in chains, somehow found their way to a microphone to sing some really great music on the radio and generally got into all sorts of fun trouble. In modern times, a felon would probably start a runaway attempt in an easily identifiable orange coverall. A runaway felon, whether in modern times or in the Great Depression era, would be easy to recognize.
But recognizing a runaway pitch trim while in flight in a King Air is a whole other matter. When the trim wheel progressively turns in one direction unabated and uncommanded, some pilots have a poor record of handling that emergency. I bet 50% of my recurrent training clients do not quickly recognize a pitch trim runaway when administered in a training environment, and they instead attempt to remedy the problem with incorrect actions.
Here’s what the scene (of the crime?) looks like. I’m in the right seat as the instructor and the training client is in the left seat. I simply take my electric pitch trim switch and hold it in one direction (usually up). I then look over at the pilot flying and watch for them to do something to remove the errant pitch trim from running steadily on one direction. All they have to do is push the LRB (little red button, that’s what we called it in the U.S. Army) on the yoke, grab the pitch trim wheel and physically hold it from moving, pull the pitch trim circuit breaker, pull the autopilot circuit breaker or all of the above. When they make any such response, I immediately remove my thumb from the electric pitch trim and let them fly the airplane by hand. It’s that simple.
I bet more than 50% of my clients wait far too long to recognize the problem, try pushing buttons on the autopilot instead of grabbing the controls, and we end up in a dangerously nose-high condition with super-heavy controls from which I have to recover the airplane.
I’m also amazed at how many pilots don’t know the audible tone that sounds when an autopilot disengages. This is a tone that every pilot should be able to decipher from memory. There’s no other sound in your cockpit like the audible tone of a disengaging autopilot!
For the armchair quarterbacks out there, it seems like such an easy problem to solve, doesn’t it? Just grab the controls and push the LRB. It is that simple. But the condition must be recognized first and herein lies the problem of recognizing the runaway. The tone is a dead giveaway, and the uncommanded pitch up/down is the other. Too many pilots don’t recognize the giveaway signals.
Turn off the automation
Before we go a step further, even though it doesn’t really need to be said, I’m going to say it anyway: When the airplane is not doing what you want it to do, grab the controls and fly the #$%^* airplane! Grab the controls, turn OFF the automation by pushing the LRB, and fly the airplane! Aircraft control is always subservient to every other consideration, especially when an emergency is being handled.
We’ve come upon a time in aviation when I’m afraid that good airmanship is being supplanted by good button-pushing. We have level buttons, envelope protection, underspeed protection and a whole host of gizmos available to us that are good safety devices. Worse, we have pilots who think these gizmos can save you from any and all problems. But all these devices can fail. None work perfectly every time, and none work as well as a proficient pilot grabbing the controls and flying the airplane. And all need to be turned OFF when control of the airplane is in question.
When I administer the pitch trim runaway in training, the pilots who try to use automation to correct the flight attitude deviation are the ones who fly the airplane dangerously close to the stall. Yes, I know there’s a level button on the panel, but that button is there when the pilot is spatially disoriented. If the autopilot disengages uncommanded and the nose is doing something you didn’t command, then there is no aircraft commander unless you grab the controls and fly.
In training, it’s the pilots who grab the controls, push the LRB and fly the airplane who excel, and I suspect these are the pilots who also recover nicely from a spatial disorientation situation in the real world.
Need I say it again? When aircraft control is in question, grab the controls and push the LRB!
Show me the muscle!
In King Air models, we have two separate devices to help with pitch control: a pitch servo and a completely separate pitch trim servo.
The pitch servo is attached to the elevator cable and is used to move the elevator. It behaves exactly as you would if you were flying the airplane with your hand. The pitch servo moves the yoke forward and backward. The pitch servo literally moves the elevator in the pitch axis when the autopilot is coupled, doing your bidding as you push the buttons related to the pitch axis of the autopilot control head. Your autopilot has a whole host of different pitch modes including ALT (altitude), GA (go around), V/S (vertical speed), GP/GS (glide path/glide slope) and PTCH (pitch attitude). Some of the better autopilots also have A/S (airspeed), FLC (flight level change) and Vnav (vertical nav) modes. When you select a particular pitch mode on your autopilot, your pitch servo is the muscle that moves the elevator to fly the mode you selected.
Photo Credit: Clint Goff
What does the pitch trim servo do? The pitch trim servo relieves control pressures. Let’s say, for instance, that you were holding altitude and flying by hand when ATC commands you to slow down 30 KIAS. You’ll pull back on the power levers to reduce torque and the airplane slows down. As you slow down, you’ll have to pull back on the yoke to hold altitude, the yoke will get heavy and your biceps will become tired. So, you’ll move the pitch trim aft too (electric or manual), relieving control pressure. Pitch trim relieves control pressure, and the pitch trim servo can move the pitch trim electronically.
There are two ways to electrically manage the pitch trim servo: (1) electric trim from either of the two (pilot/copilot) yokes or (2) when the autopilot is flying, the pitch trim is moved by sensors in the pitch servo. When you move either the pilot or copilot pitch trim, you are activating the pitch trim servo, moving it in the desired direction. When the autopilot is operating, the pitch trim servo is activated in one direction (or the other) by the pitch servo.
In older non-digital King Air autopilots, the pitch trim servo is moved by one of two switches that are in the pitch servo that sense cable tension. If tension is sensed in one direction, then the pitch trim servo is activated in that direction until the pressure is relieved. In newer digital autopilots, electrical energy is measured and if excessive energy is required by the pitch servo to hold the elevator in a certain position, it will send a signal to the pitch trim servo to rotate and relieve that pressure.
Normally, this system works well. Most autopilots bolted on a King Air are good, especially the newer digital autopilots. But this entire system is electro-mechanical; the electronics can fail and mechanical devices will fail too. When they fail in the pitch trim system, they fail with flair. Usually, the failure causes a pitch trim runaway.
A runaway can occur from one of the pitch trim switches on either yoke (pilot or copilot) failing in one direction. I’ve had several happen in my years of flying. I once had a pitch trim runaway because of a failed switch on the yoke that stuck in position after being damaged by something bumping the switch. In a King Air, the trim switch can be damaged by a pilot’s foot when entering/leaving the cockpit or by bumping your coffee canister when putting it by your left foot. That switch failed in a pitch-down manner, and I lost about 500 feet of altitude before I solved the problem.
I once had a non-pilot passenger who joined me on a flight. That person had a rather heavy notebook in his hand and accidentally laid that notebook on the copilot pitch trim. The autopilot kicked off and the airplane pitched forward. I grabbed the controls and returned the airplane to level flight, and I made sure my passenger’s notebook found its way to his knee.
The switch you cannot see
The more insidious cause of a pitch trim runaway is when a pressure switch in the pitch servo is erroneously activated. When this happens, it can be surprising. The autopilot will instantly disengage, and the airplane will begin pitching either up or down, depending on which switch failed. The pitch trim wheel will be turning. This is the time to grab the controls and hit the LRB. Speed in recognition is critical as the control forces will become increasingly heavier rapidly. In fact, pushing the LRB one time might not be enough. You might need to push and hold the LRB. If that doesn’t work, find the pitch trim servo circuit breaker and pull it. You must ensure the pitch trim wheel stops turning uncommanded!
You should know where the pitch trim circuit breaker is in the cockpit of your King Air. This is one of the important circuit breakers that you should have collared for easy identification. When you have a pitch trim runaway, you’ll be very busy. Finding that circuit breaker could be a bit difficult, especially if you are flying solo. Collar it for easy identification.
King Air aircraft have a pitch trim wheel that I love; it is easily accessible and ergonomically comfortable. I use it often when hand-flying. Sometimes I’ll lay my right knee against the trim wheel and feel when those small movements occur.
But not all King Airs have a pitch trim wheel. King Air 100 models have a completely different pitch trim system. In the 100 series, the pitch trim moves the entire horizontal stabilizer, not just a trim device. And it has a backup system that operates at half-speed. So, if a pitch trim runaway happens in a King Air 100, the pilot’s response is a bit different. A 100 series pilot needs to be familiar with this system and the emergency procedures for this unique airplane.
Whatever King Air you operate, you need to fully understand the pitch trim system and be able to recognize a runaway. A runaway will include an audio tone specific to your autopilot installed and an uncommanded pitching of the airplane. That’s your cue to grab the controls and fly the airplane, push the LRB and know where the pitch trim circuit breaker is in your King Air. Hand-fly the airplane for the rest of that flight, ideally flying to a location where you can have your airplane repaired. A pitch trim runaway is no joke; it is a very dangerous situation that requires a prepared pilot.