“I hate trim.”
We were on our way back to the airfield after completing our fourth training session. My student, connected to me using Peter Memmott’s excellent JoinFS software, was flying. Bill, as we’ll call him, was a fairly taciturn chap who was quite knowledgeable but found keeping the aeroplane straight and level hard work. To an experienced pilot trimming the aeroplane comes almost as naturally as breathing and not doing so is probably the one thing most likely to impede being able to maintain a height accurately, so Bill’s indictment of the trim wheel for his travails seemed bizarre. We’d spent some time practicing the appropriate technique in the immortal ‘Effects of Controls’ lesson some time previously and I didn’t recall any issues -- in fact, he’d seemed to pick up the principles quite quickly.
Yet for some reason Bill now seemed convinced that the trim wheel was some sort of evil inconvenience I was forcing upon him. I couldn’t understand it. Surely it couldn’t be comfortable or easy to fly the aeroplane whilst constantly having to heave on the controls?
Bill is far from the only student I’ve come across who’s had difficulty with trim. Perhaps one of the most memorable ones was James, who booked a session with me as the other couple of instructors he had flown with previously were unavailable. I’d read over his record and it was extremely positive with lots of glowing comments.
The scheduled detail was Practice Forced Landings. I first suspected something was amiss when, as James completed his pre-takeoff checks, I pointed out that the trim was set rather significantly forward of the marked takeoff position.
“Oh, I normally set it here because that’s where it needs to be once I’m airborne,” he replied.
Having persuaded him to set the trim correctly, we took off and after a few minutes I started to relax -- James’ flying seemed very accurate, in line with what I’d read. After demonstrating the forced landing procedure -- pulling the throttle to idle, trimming for the best glide speed, selecting a field, running the trouble checks and flying a nicely-planned circuit, I returned the aeroplane back to 3,000 feet or so and handed over the controls.
“OK, you have an engine failure - close the throttle, please,” I instructed.
The first thing I noticed was that James seemed to be having some difficulty maintaining the glide speed. Gone was the nice smooth flying that had got us here and instead the nose was pitching up and down, the ASI needle swinging back and forth as we lurched down on what seemed to be a fairly lumpy rollercoaster.
Of course, this was making it difficult for James to work out exactly where he was aiming and I was quite glad this was all taking place in a simulator, as I am quite certain that the effect in a real aeroplane would have nauseated even the most hardened of flyers. As we wobbled through about 1,200 feet, James started a turn toward final. Sensing he was a little low, he heaved the nose up.
“I have control!” I called over the squeal of the stall warner, shoving the throttle and the yoke forward, and pushing hard on the right rudder pedal as the left wing threatened to plunge earthward. Gingerly, I eased us out of the dive and in to a climb, thinking it was odd that I was having to hold quite so much backpressure. I glanced down at the trim indicator.
Yep, it was set about two-thirds of the way forward -- more or less where one would expect it to be in cruising flight. After putting that right, I raised an eyebrow.
“James, did you trim the aeroplane for that glide?”
The reply was incredulous. “No. Should I? I didn’t know you could change the trim in the air.”
No wonder he’d had problems. The poor guy must have been heaving on his yoke all the way down to try and maintain the target airspeed. Not only that, but he’d been flying all the way through the course (and presumably all the way through his flight simming ‘career’) without ever touching the trim, save for at the point at which the checklist stated that it should be set for takeoff.
What is this trim stuff anyway?
Before we discuss how to trim properly, let’s sort out what trim is in the first place.
Trim is used by the pilot to relieve control forces. Most light single-engine aircraft are equipped only with elevator trim, but it is possible for all three primary control surfaces to be trimmable. Most multi-engined aircraft are equipped with at least a rudder trim in addition to the elevator trim, and many of those will also be equipped with aileron trim. We’ll be focussing only on the elevator for now since that is the most commonly used, but the principles apply equally to all the surfaces.
In most light aircraft, a small tab is installed on the trailing edge of the elevator. This tab is adjustable by the pilot, often by use of a wheel installed in the cockpit and connected to the tab using an arrangement of cables and pulleys, or sometimes the tab may be driven by a small electric motor controlled using switches installed on the control column. Larger aircraft may be equipped with a trimmable horizontal stabiliser -- an arrangement where, as the name suggests, instead of a small tab on the elevator the angle of the whole horizontal stabiliser may be changed. Again, in this discussion we will focus on the trim tab, but the general principle and technique is equally applicable to aircraft with a trimmable horizontal stabiliser.
The pilot uses the control column to hold the elevator in the desired position, and then operates the trim control in the appropriate direction (either nose up or nose down).
If the pilot is maintaining backpressure (and thus the elevators are deflected 'up') he trims in the 'nose up' direction. This causes the trim tab to move down, i.e. in the opposite direction to the elevator. This provides an aerodynamic force to hold the elevator in its selected position, relieving the pilot of the need to maintain force on the control column. Naturally the reverse is true if the pilot is holding forward pressure.
But don’t I need a force feedback stick to feel this stuff?
No! As long as your controls have springs that return the pitch axis to the centre when you release it, you have a force that you are pulling or pushing against. Naturally this is likely to be a somewhat lighter force than in most real aircraft (although some aircraft can have very light stick forces indeed) but nonetheless, provided you can feel the difference between holding the stick forward or aft of the centre detent and the stick being centred then you have all the feedback you need to trim the aircraft.
What is important for accurate flying is that your controls are firmly anchored to the desk and are not slipping or sliding around as you move them. If you don’t have a hardware trim wheel, mapping the trim control to easily-accessible joystick buttons is essential so that you can comfortably operate the joystick and the trim at the same time whilst keeping your eyes looking out of the windscreen rather than fiddling around with the mouse.
The first mistake many new students make is to focus on the instruments rather than the world outside. It’s easy to see why it’s tempting -- but if we look at it logically, how large is the artificial horizon compared to the real one visible through the windscreen? Which, therefore, is going to show any changes in attitude the most clearly, no matter how subtle the change?
Another problem associated with staring at the instrument panel is that there is a (realistic) lag associated with the instrument indications. The result is that people almost universally end up ‘chasing’ the indications, particularly airspeed and vertical speed, back and forth resulting in overcontrolling and wild oscillations. In more than two years of training flight sim pilots, I cannot think of a single one who did not exhibit this tendency at some stage. It is quite remarkable how easy it is to tell that a student is looking inside and not outside during a shared cockpit training session - even when the student might be literally half a planet away!
It sounds obvious, but it is essential to get one’s eyes outside of the cockpit, off the instruments and looking out at the horizon. Note the distance between the horizon and part of the aircraft -- the glareshield, the nose cowling, the top of the wet compass etc. Every so often a scan of the instruments should be completed to confirm accurate flight, but any corrections should be made by making a small adjustment to the attitude (as identified using the outside horizon) and/or power setting as appropriate, holding the new attitude and then re-scanning the instruments to see if the desired effect has been achieved rather than using the instruments to carry out the correction
Select - Hold - Trim
The trim is used to relieve control pressures. It is not used to change the pitch attitude of the aircraft! This is accomplished using the stick/yoke to move the elevator.
Gently apply pressure to the control column to select the pitch attitude that you want, referencing the distance between the horizon and your reference point on the airframe (e.g. the top of the nose cowling).
Now wait - keep your eyes outside and hold the picture exactly steady using pressure on the stick as required. Are you having to hold forward or backward pressure?
Finally, trim by applying small bursts of trim, if you are using buttons -- or if you are lucky enough to own a hardware trim wheel, smoothly roll it in the appropriate direction. Whilst you are trimming, aim to keep the pitch attitude exactly steady by varying the pressure on the control column. You will find that as you trim, less and less pressure is necessary until as if by magic you can let go altogether and the nose still hasn’t moved - now you’re in trim!
Whilst this may take a little time at first, with a little practice you’ll soon be trimming like a pro!
What could possibly go wrong?
Here are some of the most common mistakes I see students making when they’re trying to trim the aeroplane:
- Flying the aircraft with the trim. Remember, the trim is there to relieve stick forces, not to replace the elevator. Most aeroplanes respond relatively slowly if you try and use the trim to pitch the nose up or down: the result is invariably overcontrolling and an unstable flight path.
- Not holding the attitude steady whilst trimming. Remember, the aim is to trim off the pressure you are holding. Relax the pressure very gently in proportion with the rate of trim input so that the nose holds steady. Letting go of the stick altogether before the aircraft is in trim will result in inaccurate, unsteady flying.
- Trimming in response to short-term deviations, such as turbulence. This falls in to the category of ‘flying the aircraft with the trim’, really -- again, fly the aeroplane with the control column and trim only to relieve sustained control pressures required to hold a desired pitch attitude.
Keep your eyes outside. Remember, the actual horizon is a lot bigger than the artificial one! Follow the select - hold - trim mantra. Get the picture set right first, hold it there, then use the trim to relieve any control pressure you are holding. Don’t let the attitude change whilst you’re trimming, and don’t use the trim alone to change the pitch attitude! If you need to adjust the pitch, select the new attitude using the elevator, hold it there and go through the process again.
Good luck and if you have any questions or tips of your own please share them in the comments!
Edited by skelsey