We had been issued a frequency change for an air visitors management sector handoff, and I used to be simply reaching as much as make the change when the plane violently pitched down. We had been tossed towards the ceiling with free objects flying across the cabin as I grabbed the yoke and pulled again arduous. As quickly as I felt the drive on the yoke required to maneuver it, my eyes went to the trim indicator and noticed it was trimmed excessively down. With one hand pulling on the yoke and the opposite manually adjusting the trim again to impartial, we shortly regained management and stage flight. Gathering ourselves and calming down, we checked for accidents (there have been none) and accomplished the flight with the auto-trim disabled.
To many common aviation pilots, plane elevator trim techniques are sometimes thought-about secondary techniques of lesser significance in comparison with the engine and first flight controls. Checking the trim place is on preflight checklists, however that is about it. That is a outstanding disconnect, contemplating the position of trim in some pretty high-profile accidents.
Plane elevator trim techniques have performed a task in a number of catastrophic accidents starting from the latest crashes involving the Boeing 737 Max to a Piper Navajo crash in 2021; the Colgan Air crash in 2003; and even the 2011 crash on the Nationwide Championship Air Races in Reno, Nevada, that killed the pilot and 10 spectators on the bottom. Trim techniques are critical enterprise, requiring nice respect from each pilots and mechanics.
Handbook trim techniques
Handbook trim techniques are designed to assist the pilot in all flight phases, from takeoff to touchdown. In reality, one of many necessities of the elevator trim system design is the flexibility to securely management the plane by itself, in case the first elevator management system fails. That mentioned, the trim system can exert a major management drive—a drive that may turn into harmful whether it is excessively out of limits for the flight situation of the plane.
Take the instance of the “inexperienced band” on most trim techniques that signifies the secure vary of trim for takeoff. It is a requirement from FAR 23.677(a): “The pitch trim indicator have to be clearly marked with a place or vary inside which it has been demonstrated that takeoff is secure for all heart of gravity positions and every flap place authorised for takeoff.” That implies that takeoff with the trim outdoors of the inexperienced band will not be secure. I’ve seen many pilots fly approaches whereas trimming the plane far out of the inexperienced band to lighten the elevator masses for the flare. If you have to go round in a rush, you may discover that utility of full energy with out re-trimming can lead to a strong pitch-up that might induce a stall if not caught in time.
I level this out as a result of it’s one purpose that correct rigging and upkeep of the plane’s trim system is so vital. If the elevator trim is stiff, sticks, or fails for some purpose, the plane can turn into very troublesome to fly. The identical could be true if the trim tabs are too free or not correctly balanced. At finest, these circumstances will trigger extreme vibration and put on to your complete elevator and trim management system. At worst, it could possibly trigger management system flutter and whole failure.
Electrical trim and autopilot techniques
Electrical trim and autopilot-controlled trim system failures could be particularly harmful for 2 causes.
The primary purpose is as a result of electrical trim can “masks” trim techniques issues. I’ve discovered that pilots who fly plane with electrical trim not often (if in any respect) transfer the handbook trim wheel by hand. Because the electrical trim swap gives no tactile suggestions on trim operation, these pilots do not know if the trim system is stiff or binding. Their first indication of a failure within the system can be when the electrical trim motor fails to beat the underlying mechanical drawback. Because of this their first indication of an issue could also be when the issue has turn into vital to secure flight. The warning indicators could have been current for months of flying, however they might have had no concept.
The second purpose is the dreaded “trim runaway.” Trim runaway implies an uncommanded electrical trim motion to at least one excessive or one other. In reality, trim runaway is any uncommanded or undesired electrical trim motion. The well-known Boeing 737 Max maneuvering traits augmentation system failure was mainly a trim runaway failure. If not shortly recognized and corrected, trim runaway can lead to disaster for any plane.
Autopilot techniques themselves can masks issues with the trim system. Within the case of our in-flight failure, the electrical trim was slowly (and incorrectly) transferring the plane out of trim downward. The autopilot’s pitch servo was working more and more arduous in opposition to the trim to keep up the plane in stage flight. Having not seen the altering trim, I had no concept how arduous the pitch servo was working to maintain the yoke again. When the pitch drive lastly reached the restrict of the servo clutch, the servo launched and the plane pitched over violently.
My first name after touchdown was to Alan Sparks at Autopilots Central in Tulsa, Oklahoma. He (like others on the firm) has an excessive amount of expertise with points like this. He was very clear in regards to the urgency of the state of affairs: Most autopilot points do not contain critical flight danger, he defined. Trim points, alternatively, at all times get prime precedence as a result of runaway trim can have dire penalties. With Sparks’ assist, we’re ensuring it does not occur once more.
Trim failure danger administration
The keys to managing trim system danger are upkeep, preflight inspection, and remark. All the time be sure that the trim system is rigorously inspected at annual, and anytime that you just discover one thing out of the atypical. Make the trim system a focus of your preflight inspections, together with working the trim each manually and electrically (if outfitted) earlier than each flight. Test its operation in flight as effectively. The system ought to function easily, no matter airspeed or perspective. If doable, make your approaches with trim settings throughout the takeoff inexperienced arc to organize for go-arounds. If that is not doable, make re-trimming an integral a part of your go-around procedures. Lastly, make the trim setting part of your common instrument scan whereas flying, so you possibly can catch any deviations earlier than they shock you.
The trim system is the plane’s method of serving to you fly the airplane with diminished effort. With correct upkeep and monitoring, you possibly can make sure that it is at all times working with you, and by no means in opposition to you. Till subsequent time, I hope you and your households stay secure and wholesome, and I want you blue skies.