Jim Davis

TRIM – look, Ma, no hands

Image – Joe Pieterse

It’s traditional to bitch about the Regulator, but we have been lucky to have some really brilliant CAA /DCA bosses in South Africa.

I think of Barry Radley, an ex-RAF Farnborough test-pilot who taught me a very valuable lesson – basically that I knew nothing. And yet he quietly coaxed me through all my flight tests.

Perhaps my favourite was the gentle and fatherly Karl Sembach who always made his testees (yes that is the right word) feel at ease. He somehow allowed me to pass an instrument renewal in my old Apache ZS-DSC (Dog Shit Charlie) despite flying him on to a collision course with George Peak.

But I want to tell you about the great Robbie Robinson who sadly died recently. Robbie made every test a learning experience, and his favourite exercise was the steep turn.

We were doing a SAAF wings test and I remember struggling round a 60 degree banked turn in an Aztec over the Cape Recife lighthouse on the southern tip of Nelson Mandela Bay.

Struggling because I hadn’t practiced, and because you don’t have a nose to watch in a twin, and because the Aztec needs a fair amount of physical muscle to hold her at that angle without losing height.

“Are you comfortable?” says Robbie.

“Grunt” says I.

“Why don’t you trim?”

“Am I allowed to?”

“Who said you’re not?”

So I grab the overhead window winder and give it a couple of twirls which takes much of the pressure off the stick.

“Okay, I’ve got her” says Robbie as he takes over and gives the trim another couple of turns.

Then he takes his hands off and sits back smiling as we sail round and round with our hands in our laps.

That was one of Robbie’s favourite tricks – he had this thing about trimming – in fact he had a trim wheel at home which he wanted to be buried with him. He said that if things got too warm he would trim up slightly until it cooled off.

Before one test he showed me the back page of the form. He had signed it and written PASS. He just smiled and said, “Don’t disappoint me.”

He was really saying, “I have confidence in you – now let’s go and have fun.” What a wonderful instructor.

All that is an intro to say that if Robbie thought so highly of trimming, then maybe we should all take it seriously.

Actually, my first two experiences of trimming were vastly different. I did all my PPL training in a 65hp Piper J3 Cub. I learned that the trim needed to be set neutral before takeoff if you had two on board – and there was another mark for solo flying.

We didn’t adjust it round the circuit because we didn’t need to. We were either climbing, or in level flight, or in a glide, for such a short time it seemed unnecessary. Dirty Bossie, our instructor, would shout at me not to fiddle with things.

“Hell”, he explained, “you have a stick, a rudder and a throttle – we don’t want to fry your little brain with extra stuff, do we?”

I went straight from being in command of an aeroplane with five instruments to being unofficial co-pilot on a Comanche with about 300 instruments. Whenever we went anywhere, my boss, Old Piet van der Woude would take off, climb and level off and then spend the next few minutes making minute adjustments on the overhead trim handle.

It was a delicate process that called for infinite patience. The idea was to get the aircraft ‘on the step’, which I now know is nonsense, but at the time it was an elusive condition over which God granted dominion only to the world’s top pilots.

Old Piet would eventually get the nose tucked down and the Comanche going like an express train. He would hand over to me and we would lose 10 mph almost instantly. This would cause a thinly restrained air of hostility in the cockpit for the rest of the flight.

So from Piet, I learned that the main purpose of the trim was to get the aircraft on the imaginary step.

Finally Robbie taught me that trimming properly is the hallmark of a good pilot. The aircraft should always be trimmed – hands off. But one is allowed a few seconds of stick pressure during takeoff and landing.

As an instructor you will need to differentiate between levelling off in the circuit and doing so at the top of climb on a cross-country.

In the circuit, a new pilot simply doesn’t have time to trim hands-off for each leg. The best they can do is to take most of the load off the stick for most of the time.

Perhaps strangely, I advise you not to have your pupe use the rudder trim in the circuit. I would much rather see her making a habit of using her feet to counteract power changes. So start with the rudder trim neutral and chase her to use right rudder in the climb and left rudder in the descent.

Of course during a long climb out to the GF or descent on the way home, sure thing – get her to trim the rudder as well as the elevator.

And in line with my belief that the more you know about the aeroplane the better and safer pilot you will be, you – as her flying guru – need to understand how the trim works on every aircraft you fly. Not sure that a knowledge of the trim system can be a life saver? Let me give you two examples.

One of Placo’s engineers took off from Plett in a 250 Comanche. Soon after takeoff the elevator went sloppy in his hands – easing back on the stick did nothing. The elevator cable had broken. He throttled back gently and the aircraft landed in open country straight ahead without too much damage.

Could he have done better?

Indeed he could. He could have flown it safely round the circuit and landed normally. But only if he had time to think it through.

When I said the elevator cable had broken – he would have known it actually has two elevator cables – one up cable and one down cable. Only the up one had broken. This means that had he trimmed up, he could have flown and landed by simply using forward pressure on the stick.

Here’s the other one. My friend Bob Ewing came within inches of being killed when the elevator of a Yak 52 jammed. He had been doing negative G aerobatics. As he tried to pull out of a loop, the stick locked up solid – it simply would not move back.

Luckily Bob was near Bisho which has a nice long tar runway. He found that if he maintained 120 mph the nose would be about level, but if he throttled back, even slightly, she would pitch down.

He didn’t dare to lower the undercarriage, partly because it was too fast, but mainly because it would pull the nose down.

With a lot of skill Bob managed to land on the runway at that speed, with the gear up. He and the pupe struggled out of the wreck before it caught fire.

Could he have done better?

Actually yes.

The aircraft has a standard trim tab, so if the elevator can’t move, the trim then becomes a mini elevator – however it works in the wrong sense.

Trimming down would normally cause the trim tab to move up and this will lower the trailing edge of the elevator. That causes more lift from the tail plane which pitches the nose down.

But now, with the elevator jammed, trimming down will still move the trim tab up so the tab works like the elevator – it moves the tail down which raises the nose.

So if Bob had worked this out beforehand he could have flown the aircraft to quite a large extent, and possibly even landed at a sensible speed.

So I say again, understanding the machinery can be life saving.

Would this have worked on an aircraft that uses springs or bungees to relieve stick pressures? Show your pupe different aircraft and discuss the trim systems on a Mooney and a Piper Cub and a Bonanza, and a Cherokee. Remember, telling her doesn’t do the job – you have to involve her.

Be very sure that you understand the Cherokee and Comanche systems before you start explaining them – they are a bit tricky but extremely efficient.

Incidentally, Bob’s problem was that the little white knob that locks the canopy had been lost at the last MPI. It had dropped down into the guts of the aircraft, rolled back and somehow jammed the elevator when they were inverted.

Okay enough already about trim. Now let’s have a look at flaps.


Do you remember how Earnest Gann nearly flew a Liberator into the Taj Mahal? Don’t even hint that you haven’t read ‘Fate Is The Hunter.’ It’s mandatory pre-solo reading and then reading again every two years – it’s a standard part of license renewal.

Anyhow just in case you have forgotten this story here are Gann’s words:

While most airports in the world are surrounded by industrial junk, Agra is fantastically endowed. There is a border of dark green trees along the northern side, and just beyond the trees there is the Jumna River.

Almost on the shore of this river, stands a monument to love – the Taj Mahal. Its dome and delicate minarets are just visible beyond the trees, for the runway points at them straight as a cannon.

 We have completed the pre-take-off check of the C-87 and are agreeably surprised to find all in order. It is unbearably hot, the true torch-heat of India, and I have duly considered it because no aeroplane wing exerts the proper amount of lift in hot air.

The very factor of flight will diminish in direct relation to the increase in temperature. Ahead the runway wriggles in the heat waves and appears foreshortened. Buzzards wheel against the blank and garish yellow sky and the only relief to our eyes is the black line of trees marking the northern limit of the field.

I will bear those trees in mind… Just over their tops there will be a layer of cooler air, which will be descending like an invisible waterfall. I will gain as much altitude as possible before reaching the trees, knowing some of it will be lost in passing.

I release the brakes and goad the C-87 down the runway at full throttle.

It is a ponderous, dream-like business at first, but this is always so… Such thoughts are fleeting because I already sense something is wrong. We are halfway down the runway and have only achieved sixty miles an hour. I glance quickly all around—the instruments, the engines, and the remainder of the runway.

What the hell is wrong now? Even this C-87 has never behaved in such a leisurely fashion.

Eighty miles an hour. We need one hundred and twenty and I should prefer one hundred and thirty. The trees dance towards us, wavering in the sun. Ninety. The choice is gone, other than a certain plunging through the trees. One hundred. I haul back tentatively on the elevator controls seeking response. Very mushy.

A glance at the engine-head temperatures and a quick resolve not to look again. With their task less than half done the engines are already far beyond their allowable heat… One hundred and ten at last. I can raise the nose wheel a little, but not yet enough. We just cannot clear those trees. But we must try… I haul back on the controls. The C-87 leaves the ground, sinks back, bounces on one wheel, then staggers aloft in a mushing half stall. 

The trees are no longer there, but here. We clear them. I can count the leaves. A flock of buzzards explodes before us. We sink back towards the trees and are going to hit. The trees are a thin fringe along the river. Our tail is just past them as we sink below their tops. We are for an instant in the clear, over the river. Full power. Air speed one hundred and thirty and still sinking. 

Now, a new obstruction, dead ahead. The Taj Mahal. They are making repairs. Much of it is covered with scaffolding and I can see the workmen moving about. I can see the folds in their turbans. I can see their mouths open as we approach. I cannot see any beauty. The quickest and surest way to finalize a semi-stall in an airplane is to turn it.

But I must turn or they will have much more repair work to do on the Taj Mahal. 

There is one crazy hope. It is not written in any book of aerodynamics.

“Franko! Full flaps!” He slams down the lever. The C-87 collides with a soft invisible wall. The air speed falls off and everything shudders. But we balloon upward a hundred feet almost instantly. Enough to barely clear the spike of the first minaret. “Now ease them up, slowly!” Franko complies and we sink again. But speed is returning. And I think we can clear the next minaret without turning. It flashes past. I see a group of workmen cringe against the scaffolding. The Taj Mahal is gone. We swoop down beyond it and with agonized slowness begin picking up enough speed for a halting climb. It had all taken less than twenty seconds. In that space of time I had grown much older. Say, in wear and tear, some several years.

The reason he nearly ruined the Temple of Love is that refuellers gave them twice as much fuel as they had asked for. And Gann didn’t notice.

But his knowledge of aerodynamics saved them from a fiery death.

Full flaps can briefly give you enough lift to hoick you over nasties. They use your inertia to convert airspeed to lift.

But after the flaps have done their rescue job, they leave you with an airspeed problem. You must have room to shove the nose down and gently bleed off the flaps.

Now imagine you are doing a forced landing and you can see you will be 50m short in your flapless condition.

Banging on full flap at the last second will stretch your glide, or hop you over the fence. Think about it – you were about to stall into the rocks at 70 mph. But full flap will delay the stall by 15 mph and allow you to fly the last bit.

It’s an inertia thing – the heavier the aircraft the more effective it is.

If this is not clear in your mind then think about it until you can explain it to your pupe. Don’t ask another instructor – they probably don’t know. Go fly and try it on your own.

The tales about Robbie Robinson’s trim wheel, Bob Ewing’s jammed elevator, and Ernest Gann’s unconventional use of flap, are not just stories – they are here to make sure you and your pupe remember.

I don’t want you to be an instructor – I want you to be the best instructor you can be.


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