Holding Altitude
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Holding Altitude
I have a skills test coming up soon and would appreciate some advice and/or tips on holding altitude accurately. I have found this to be the most difficult aspect of flying since starting training.
When flying in turbulence it gets even worse and my altitude can vary by several hundred feet. In controlled airspace I have to make a conscious effort to monitor the altimeter and VSI on a regular basis to avoid infringements.
I believe I'm trimming effectively so is this just a learning curve and is likely to be a show stopper in the skills test?
When flying in turbulence it gets even worse and my altitude can vary by several hundred feet. In controlled airspace I have to make a conscious effort to monitor the altimeter and VSI on a regular basis to avoid infringements.
I believe I'm trimming effectively so is this just a learning curve and is likely to be a show stopper in the skills test?
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I think we've all come across this at some point. I know I have!
Usually when the aircraft is hunting in vertical speed, that is a sign that the aircraft is incorrectly trimmed. You may think you have trimmed it out OK, but it won't be.
Check the obvious things. Is the attitude right? Make reference to the horizon using the top of the instrument panel in S&L flight. How high above the top of the panel should the horizon be? Is the attitude correct? That's the first thing.
Secondly, the airspeed. If you've trimmed out for 90Kts but after hitting a thermal you've slowed to 85Kt or accelerated to 95Kt, then the the aircraft is no longer in trim. If you're hunting in airspeed then that's a sign you are incorrectly trimmed.
Third, check the power setting. Do you have normal cruise power set? Is the carb heat on? Have you adjusted the mixture?
I was always taught PAT: Power, Attitude, Trim. When you trim you are trimming for a certain attitude at a particular airspeed and a set power setting. If one changes then it's back to square one and you'll need to re-trim!
Hope this helps. Some PPRuNE-dwelling instructors may care to elaborate? (Apologies if my attempt at explaining is mince; perhaps some FI patter may help!)
Smithy
Usually when the aircraft is hunting in vertical speed, that is a sign that the aircraft is incorrectly trimmed. You may think you have trimmed it out OK, but it won't be.
Check the obvious things. Is the attitude right? Make reference to the horizon using the top of the instrument panel in S&L flight. How high above the top of the panel should the horizon be? Is the attitude correct? That's the first thing.
Secondly, the airspeed. If you've trimmed out for 90Kts but after hitting a thermal you've slowed to 85Kt or accelerated to 95Kt, then the the aircraft is no longer in trim. If you're hunting in airspeed then that's a sign you are incorrectly trimmed.
Third, check the power setting. Do you have normal cruise power set? Is the carb heat on? Have you adjusted the mixture?
I was always taught PAT: Power, Attitude, Trim. When you trim you are trimming for a certain attitude at a particular airspeed and a set power setting. If one changes then it's back to square one and you'll need to re-trim!
Hope this helps. Some PPRuNE-dwelling instructors may care to elaborate? (Apologies if my attempt at explaining is mince; perhaps some FI patter may help!)
Smithy
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If you are flying VFR and controlled airspace is not an issue and you are out of the circuit then it is not necessary to keep a constant altitude. If you have trimmed correctly and you gain or loose height due to turbulence don't worry about it - the aircraft will settle itself again after a while, as long as you are not constantly chasing the airspeed or the altitude.
On the other hand, if you are in a situation where you do need to keep a constant height, then adjust your height immediately. Don't wait. Be careful not to over-correct.
On the other hand, if you are in a situation where you do need to keep a constant height, then adjust your height immediately. Don't wait. Be careful not to over-correct.
and would appreciate some advice and/or tips on holding altitude accurately
It takes quite a while to regain altitude which tells you that it also takes quite a while to deviate from it, so you are not picking it up and acting on it soon enough.
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so you are not picking it up and acting on it soon enough.
This makes sense. In retrospect I spent the first few hours fixating on the instruments with my instructor imploring me to look outside. Maybe I've turned full circle and need to incorporate an instrument scan on a more frequent basis.
I'll need to practise this a bit more as the skills test is August 4th and I believe the tolerance is 100 feet or so - this is the bit that worries me.
This makes sense. In retrospect I spent the first few hours fixating on the instruments with my instructor imploring me to look outside. Maybe I've turned full circle and need to incorporate an instrument scan on a more frequent basis.
I'll need to practise this a bit more as the skills test is August 4th and I believe the tolerance is 100 feet or so - this is the bit that worries me.
As was pointed out earlier trim is secret to holding a constant altitude. But before you trim the aircraft has to be in equalibrium, that is not climbing or descending (level flight attitude) and at the right airspeed (correct power setting). I find many new pilots rush the trim and never really get the airplane properly trimmed so they end up chasing the altitude because the aircraft will climb or descend as soon as the pilot becomes occupied with any other flying duty. With respect to Molesworth 1's comment about holding altitude not beeing important....I could not disagree more. Holding altitude is like every other flying skill, you get better with practice. If you make an effort to fly accurate altitudes you will quickly find it becomes easier and easier.
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Best test to see if you've trimmed properly: Let go of the stick/yoke. Not just for a few seconds, but for five minutes or so. Watch the altimeter for any trends and trim in tiny, tiny increments, then watch the trend again. At the same time, small bank excursions can be corrected with the rudder. But try to keep your hands off the stick/yoke the whole time.
It will not work in turbulence and it's also a bit too coarse a method to apply in controlled airspace, but it's a very good training exercise. If you have height excursions several 100 feet up or down it's not just turbulence or not paying attention, it's also overcorrecting and improper trimming.
The other thing that you might be doing wrong is leveling off after the climb. Interesting piece about this in last months Flyer magazine. After leveling off, maintain full power and let the aircraft accelerate to cruise speed (assuming cruise speed is below redline RPM). Only then select cruise RPM, and trim. If you set cruise RPM immediately after leveling off, but while the aircraft is still accelerating, you'll be hunting the correct power setting and trim for a long time.
It will not work in turbulence and it's also a bit too coarse a method to apply in controlled airspace, but it's a very good training exercise. If you have height excursions several 100 feet up or down it's not just turbulence or not paying attention, it's also overcorrecting and improper trimming.
The other thing that you might be doing wrong is leveling off after the climb. Interesting piece about this in last months Flyer magazine. After leveling off, maintain full power and let the aircraft accelerate to cruise speed (assuming cruise speed is below redline RPM). Only then select cruise RPM, and trim. If you set cruise RPM immediately after leveling off, but while the aircraft is still accelerating, you'll be hunting the correct power setting and trim for a long time.
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Trim, trim, trim 
Should be taught in the PPL but often isn't.
But to be fair, it is hard to hold altitude if flying in turbulence. It's hard work, and vertical excursions are accepted - so long as you are not flying 100ft below the base of CAS
In VFR, OCAS, altitude is not important legally, but one should aim to fly accurately in general. Then, when progressing to instrument flight, it becomes a lot easier.
Should be taught in the PPL but often isn't.But to be fair, it is hard to hold altitude if flying in turbulence. It's hard work, and vertical excursions are accepted - so long as you are not flying 100ft below the base of CAS
In VFR, OCAS, altitude is not important legally, but one should aim to fly accurately in general. Then, when progressing to instrument flight, it becomes a lot easier.
I agree that one should certainly try to keep within +/- 100 feet. However, I also agree that it's not always easy. Today there was no significant turbulence, but flying at around 3000 feet below broken cloud at around 6000 feet, I had to watch the altitude all the time. I'd get it perfectly trimmed at normal power and speed and it would be alright for 30 seconds, then the altitude would start to drift up or down. I'd probably flown into slightly warmer or cooler air, or into a slight up-draft or down-draft, and now it was out of trim. I had to keep re-trimming. And that wasn't in any perceptible turbulence.
So to say:
is true, but it may have been correctly trimmed a few seconds ago!
So to say:
Usually when the aircraft is hunting in vertical speed, that is a sign that the aircraft is incorrectly trimmed.
As others have said, trimming properly is one of the key skills to accurate - and relatively workload free - flying. Lots of pilots try to finalize the trim setting before the aircraft has stabilised. Fair enough to wind in some trim to relieve the control pressure while it is still accelerating but don't confuse this with a final trim setting. The aircraft must have stabilised at a constant speed with power set before a final trim can be done.
Assuming your scan to monitor performance is OK, how tightly are you holding on to the control column? Your grip affects your ability to trim to 'hands off' - the tighter you grip the less you're able to feel whether you're still holding pressure on the control column. The goal is to hold the column like a lightly gripped pencil. Even better if you can hold your forefinger & thumb far enough apart so that only one of them can be in contact with the column at any time. Try using a grip with your 2nd, 3rd & 4th fingers tucked into your palm, leaving only your 1st finger & thumb to grip the column - then spread those digits far enough apart so that they can't both touch the column at the same time. You'll soon see if you're having to push or pull on the column to hold an attitude. Thumb to push, 1st finger to pull and avoid squeezing the column between them.
At times you will need more pressure on the column than either of those two digits can provide, so use more fingers to pull, or the heel of your thumb/palm of your hand to push. Return to a single digit pushing or pulling as soon as trim state allows. Remember not to squeeze though.
Assuming your scan to monitor performance is OK, how tightly are you holding on to the control column? Your grip affects your ability to trim to 'hands off' - the tighter you grip the less you're able to feel whether you're still holding pressure on the control column. The goal is to hold the column like a lightly gripped pencil. Even better if you can hold your forefinger & thumb far enough apart so that only one of them can be in contact with the column at any time. Try using a grip with your 2nd, 3rd & 4th fingers tucked into your palm, leaving only your 1st finger & thumb to grip the column - then spread those digits far enough apart so that they can't both touch the column at the same time. You'll soon see if you're having to push or pull on the column to hold an attitude. Thumb to push, 1st finger to pull and avoid squeezing the column between them.
At times you will need more pressure on the column than either of those two digits can provide, so use more fingers to pull, or the heel of your thumb/palm of your hand to push. Return to a single digit pushing or pulling as soon as trim state allows. Remember not to squeeze though.
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Power, attitude, trim....Fine, but you have to settle your power and attitude first, all to easy to 'hunt' the power and attitude, i.e you increase power, or decrease power and attitude before you allow it to stabilise, once stabilised, trim to maintain. If in turbulence, then just adjust as necessary using the stick (or yoke), if it's heavy turbulence, let it ride, just keep your wings level and watch your airspeed.
PAT Power, Attitude, Trim. as has been said, for everything i.e. starting a descent, levelling off after a descent, starting a climb EXCEPT levelling off after a climb, when it's APT.
Think about how you would use the power ( accelerator ) in a car to maintain a constant road speed up and down a series of hills.
Do nothing in a hurry, make a small change and wait, don't 'chase' airspeed or height - get the attitude right.
Find a rivet, or corner of the windscreen, that matches the horizon in a turn, and look ahead of the nose in the direction of turn, but out of the corner of your eye see the rivet you have picked and pull it towards where your eyes are looking - couse, you'll need a different rivet - or whatever - for left and right turns ! Relax your iron grip on the control column.
Think about how you would use the power ( accelerator ) in a car to maintain a constant road speed up and down a series of hills.
Do nothing in a hurry, make a small change and wait, don't 'chase' airspeed or height - get the attitude right.
Find a rivet, or corner of the windscreen, that matches the horizon in a turn, and look ahead of the nose in the direction of turn, but out of the corner of your eye see the rivet you have picked and pull it towards where your eyes are looking - couse, you'll need a different rivet - or whatever - for left and right turns ! Relax your iron grip on the control column.
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I'd probably flown into slightly warmer or cooler air, or into a slight up-draft or down-draft, and now it was out of trim. I had to keep re-trimming.
Trim, in a light aircraft, essentially determines the speed. The power setting determines whether you go up or down, or stay level, with that speed. That's simply the way the balance of forces works out.
When you enter an up- or downdraft the balance of forces is still the same so you're technically still "in trim", but you're just climbing with the air, or descending with it. If you want to prevent that, adjust power a bit and use the stick to help the aircraft settle a bit earlier in its new attitude, or use the stick to force the aircraft to climb or descend without changing power and accept a speed variation.
Retrimming in this situation should not be necessary, assuming the up- and downdrafts are temporary. Because once you exit the up- or downdrafts you'd otherwise have to trim again for a stable cruise.
And that's the whole idea about trim: you trim for the stable situation. The stable climb, the stable cruise or the stable descent. Anything temporarily is handled by accepting some stick forces without retrimming. You also don't trim while making a turn, do you?
Having said that, chasing the correct altitude, attitude and airspeed in turbulence can be really tiring and uncomfortable. If you're in controlled airspace and have an altitude assigned, or near controlled airspace and worried about descending through the base, or something like that, you have no choice. But if you're out there in the blue wild yonder it is far less stressful, and more comfortable for your passengers, just to accept the occasional altitude excursion as long as it's not too severe. In the end the up- and downdrafts will probably cancel each other out anyway.
Retrimming in this situation should not be necessary, assuming the up- and downdrafts are temporary. Because once you exit the up- or downdrafts you'd otherwise have to trim again for a stable cruise.
But if you're out there in the blue wild yonder it is far less stressful, and more comfortable for your passengers, just to accept the occasional altitude excursion as long as it's not too severe. In the end the up- and downdrafts will probably cancel each other out anyway.
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Tero...Plenty of good advice above, just to say that if you are in CAS, you should advise the controller that you are in turbulent conditions, But, if you're taking your skills test, I doubt you will experience anything other than mild turbulence, in which case forget trim because it's useless in such conditions, just fly the plane using stick(yoke) to maintain altitude, as has been said before, small inputs, you don't want to overstress the airframe, just keep it as close as you can to maintain approximate altitude, but above all, keep an eye on your airspeed, I don't think you will experience anything as medium or severe turbulence on a skills test, but worth bearing in mind for the future.
Good luck with your skills test.
P.S. As FREDAcheck says, you should be able, in mild turbulence, you should be able to keep within +/- 100 ft, forget the trim, just fly the plane.
Good luck with your skills test.
P.S. As FREDAcheck says, you should be able, in mild turbulence, you should be able to keep within +/- 100 ft, forget the trim, just fly the plane.
Last edited by Lightning6; 27th Jul 2009 at 02:06. Reason: Added text.
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Do not keep retrimming. Set the trim for a given power speed. Doing it even in turbulence becomes second nature.
It maybe worth watching what happens when you have the aircraft on autopilot with alt hold in moving air.
Watch the VSI and the ASI ON on autopilot. You will notice as you hit rising air the VSI will indicate a slight climb. The aircraft will compensate by trying to hold altitude by pitch forward. As it does so the speed increases on the ASI which further increases the desire for the aircraft to climb.
Watch what happens in sinking air. The VSI indicates that sink! the auto tries to hold altitude with the result that the aircraft pitches up and the airspeed declines. ETC ETC ETC.
You could not possibly be trimming for every movement in the air flying manually so trim it for still air and then command the aircraft to hold altitude with small amounts of pressure on the control column.
Scan out of the aircraft but also watch for trends on the VSI the altimeter and the ASI.
If the VSI starts to show a trend followed by a movement up on the Altimeter command the aircraft back onto the altitude and hold it there. It is a matter of slight pressure on the column if you have the aircraft trimmed correctly for still air.
Pace
It maybe worth watching what happens when you have the aircraft on autopilot with alt hold in moving air.
Watch the VSI and the ASI ON on autopilot. You will notice as you hit rising air the VSI will indicate a slight climb. The aircraft will compensate by trying to hold altitude by pitch forward. As it does so the speed increases on the ASI which further increases the desire for the aircraft to climb.
Watch what happens in sinking air. The VSI indicates that sink! the auto tries to hold altitude with the result that the aircraft pitches up and the airspeed declines. ETC ETC ETC.
You could not possibly be trimming for every movement in the air flying manually so trim it for still air and then command the aircraft to hold altitude with small amounts of pressure on the control column.
Scan out of the aircraft but also watch for trends on the VSI the altimeter and the ASI.
If the VSI starts to show a trend followed by a movement up on the Altimeter command the aircraft back onto the altitude and hold it there. It is a matter of slight pressure on the column if you have the aircraft trimmed correctly for still air.
Pace
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Thanks for all the good advice and encouragement. The skills test is next Tuesday and the weekend will be spent doing some revision with my instructor. I'll post the (hopefully positive) result next week.
Tero
Tero
