Glide descent leveling off technique [Archive]

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shumway76

18th Nov 2011, 23:11

GeeWhizz

18th Nov 2011, 23:24

All seems a bit complicated and jiggly to me. What with full power, attitude changes, cruise power etc. Perhaps it's type dependent, but in C150/152/172 and the PA28s I stick to the P-A-T model. As I'm approaching the level I want I add power to cruise, raise the nose, and trim.

Are there many instances when a full glide descent to a specified level is necessary? PFLs involve bursts of power for engine warming, and glide approach to either land or touch and go. Otherwise the P-A-T model should be ample unless type/POH requires another method.

Pilot DAR

19th Nov 2011, 01:39

Yes, complicated... Perhaps advanced - turbine - aircraft respond well to this type of handling, but a light GA aircraft not so much. Obviously there are reasons for glide descents, though whoever pays for the engine cylinders will thank you for keeping such flying to a minimum. Shock cooling is not good for the cylinders. Easy on the power changes, particularly on cooler days....

Consider feeling what the airplane is trying to tell you more, and watching the airspeed less. When you are aware of what the plane is telling you, you're even more ready for other types. On the other hand if you are flying from a recipe of numbers and settings, you're going to miss some of the basics. If you have not done it yet in your flying, it should happen for you soon, that you fly entire circuits with some of the instruments (particularly the airspeed) obscured, just to make you feel rather than read your flying.

A gliding approach, when considering the care of engine, is excellent practice for forced approaches, and power off landings. Terminating a glide with full power is really ineffective energy management - Sort of like jack rabbit driving. I'm sure that sailplane pilots here would have lots to say about such techniques.

Fly as though you're trying to impress the owner of the aircraft with how in tune you are with it, and how gently and gracefully you'll treat it - Uncle Sam, on the other hand, is not so sensitive to this, so the Air Force probably has other techniques....

172_driver

19th Nov 2011, 02:03

And I would suggest starting to re-trim the aircraft as soon as you've given full power. Coarse trimming! The plane can be a handful otherwise, especially for a new student trying to maintain a straight & level attitude. Then you can fine tune once the cruise power is set.

shumway76

19th Nov 2011, 02:15

Thanks for the info. I tend to disagree with the ex-airforce instructor. Can't use the "slightly above s&l attitude then progressively lower to s&l" in a C172, as from 70kts, when you apply full power, the response is real quick.

As for the full power from glide descent, well, of course it's done smoothly, not rammed.

The way we are thought about reducing & increasing airspeed is as follows:
When an airspeed change of more than 20kts is desired, use the full power / idle power initally , then select the proper RPM 5kts from desired airspeed.
eg. Aircraft at 70kts, you want to accel to 100kts. Select full power (eg. 2400RPM), as airspeed increases, at 95kts, decrease power to cruise at 100kts (eg. 2200RPM)

GeeWhizz

19th Nov 2011, 02:55

Adding power in a piston aeroplane will always result in a quick response; this much fuel/air = this much rpm = this amount of pull. I've sat and watched many jets spool up to a higher power 10 seconds in anticipation. Piston props don't need it; I can't speak for turboprops but at a guess it'd be somewhere in between.

eg. Aircraft at 70kts, you want to accel to 100kts. Select full power (eg. 2400RPM), as airspeed increases, at 95kts, decrease power to cruise at 100kts (eg. 2200RPM)

I'm not sure I understand the purpose of this method of changing speeds. You clearly know approximate power settings that achieve certain speeds. Therefore my method is exactly that. Select the power value I expect to maintain the desired speed, allow the ASI to settle, readjust the power as required for accuracy.

I'm also not sure of the situation you might do this. Acelerating and decelerating between 70kts and 100kts in a C172 is a challenge in itself whilst flying straight and level. My experiences of this are instructors assessing my ability to control the aeroplane whether visually or by instruments - control and scan exercises.

Now I am sure there cannot be many variants of the C172 that will maintain 100kts at 2200rpm ;)

shumway76

19th Nov 2011, 06:09

It's off topic, but in reply to the why change power from 70kts to 100kts from idle to full power:

An airspeed change of greater than 20kts technique:
Let's say you're at 1800RPM, 70kts s&l. To increase speed to 100kts in s&l you increase to full power, then anticipate 5kts before 100kts (at 95kts) select 2200rpm (which is supposed to give you 100kts s&l).
The reason of this extra burst of power before reducing it is to make the airspeed change faster.

Pace

19th Nov 2011, 06:48

i dont believe this is an ex RAF method as its totally stupid! Why would you be descending at below cruise speed?
If anything you will be descending above cruise and watching the speed decay as you level and advance the Throttels back to cruise.

You have two sources of energy in the descent! The potential energy in your engine but more important in the descend you have the energy you are tapping into inherant in the airframe.

If you like you have surplus available in the descent hence the problem is one of going too fast not too slow.
Consider also its bad practice to descend a piston on a closed throttle and this theory makes no sense at all.

Consider also your PAX do they want big burts of power? No they want a smooth flight.

The only time you may want to come down slowly is more in IMC flight where you are entering heavy cloud or thunderstorm or in area of turbulence and are trying to protect the aircraft from structural damage but that is relevant to the cruise as well as descent.

Pace

GeeWhizz

19th Nov 2011, 07:51

During a glide descent it is possible that the speed would be less that cruise. Glide performance for PFLs / approaches usually means about 20kts slower than the cruise, somewhere around powered approach speed.

Lastly, it may change the airspeed faster but at what cost? Control: many trim changes, pushing and pulling on the column. Uncomfortable for the pilot leaving out the passengers!

But If it works I guess :ok:

fireflybob

19th Nov 2011, 08:03

Why full power? Why not select cruise power - ok fixed pitch propeller you'll have to set a tad more (50 rpm?) as the rpm will increase slightly as the speed increases.

Yes the initial attitude required will be that which is appropriate to fly at 70 kts - as the speed increases you will need to progressively lower the nose to maintain height (technically the student should know how to do this as he has already been taught straight and level flight at different airspeeds, hasn't he?).

Would teach a coarse trim initially and then fine trim as the speed stabilises.

An airspeed change of greater than 20kts technique:
Let's say you're at 1800RPM, 70kts s&l. To increase speed to 100kts in s&l you increase to full power, then anticipate 5kts before 100kts (at 95kts) select 2200rpm (which is supposed to give you 100kts s&l).
The reason of this extra burst of power before reducing it is to make the airspeed change faster.

shumway, agree but that assumes that you want to achieve target airspeed as quickly as possible - I believe CFS teach both techniques - ie slow/fast acceleration (and also slow/fast deceleration)

IO540

19th Nov 2011, 08:16

Reselecting cruise power should make the aircraft level off. Nothing else should be needed.

thing

19th Nov 2011, 08:22

Reselecting cruise power should make the aircraft level off. Nothing else should be needed.

I was thinking that but don't feel qualified to make those sort of comments.

Pace

19th Nov 2011, 08:38

During a glide descent it is possible that the speed would be less that cruise. Glide performance for PFLs / approaches usually means about 20kts slower than the cruise, somewhere around powered approach speed

GeeWhizz

Read his opening piece! He is not talking about the final approach phase of the flight or a forced landing where he aint going to level for long anyway or have any power to do that leveling! but a typical descent and leveling at ones chosen altitude.

Obviously when you are landing you will come back to your reference speed with flap, gear etc where you do that is questionable?

I know pilots who will trundle in from 6 miles out causing a massive traffic jam while they stick to their 70kts blind to conditions or wind shear:ugh:

I know others who will fly faster speed and reduce to their reference speed within the last 200 to 500 feet.

I was never a supporter of the closed throttle glide approach anyway and never understood the benefits of such an approach! Infact there are many inherant dangers in such approaches.

But he is talking about a normal descent and level to a chosen altitude!
Other than turbulent air there is no reason to be below CS.
Sometimes I wonder if we concentrate to much on bits in training rather than a much broader appreciation of energy and drag management?

Pace

Pilot DAR

19th Nov 2011, 14:12

As for the full power from glide descent, well, of course it's done smoothly, not rammed

Though the 172 does not have defined "Abnormal Procedures" that it what doing this would be. Under most atmospheric conditions this can be done with no harm in a 172, but that does not make it goo practice in a 172, or any other piston powered aircraft. If you do this in a turbo 210, the owner is going to rap your knuckles!

If a pilot has mismanaged their descent so much that glide descents are required, that's simply poor planning. Back in my turbo Aztec days, the requirement for descent power was to decrease by 1 inch of manifold pressure (which would roughly equate to 100 RPM - fixed pitch) per minute. A faster power change was considered potentially harmful to the cylinders, and an abnormal procedure.

Though turbine powered aircraft, which would include the military pilot's PC7 are capable of rapid power changes without harm, that does make it "good", smooth, or appropriate technique. Military aircraft and training are aimed at a very different role for the aircraft, and "smooth" and caring to the aircraft may not be elements of that training or technique.

IF, a power off glide is necessary because of botched planning, or some type of quasi emergency, then yes, flying the resulting glide at a slower airspeed is better for the engine, that rocketing down at idle power and cruise airspeed. Damage from shock cooling is very real, occurs in seconds, and is very expensive to repair. I once had to rapidly descent a Cessna turbo 207 because I had spotted a boat in distress on the lake. Though I closed the cowl flaps, and was still "gentle" with the power reduction to idle, and slowed the descent speed (reduces undesired cooling) I was stunned at the incredible rate of cylinder cooling indicated by the cylinder head temp indicator. I immediately though back to the 1 inch of MP per minute, and was reminded of the wisdom of that.

All that being said, as I read a lot of the foregoing, I see lots and lots of numbers, but little sense that the pilots are simply flying the plane. Is this a result of pilots spending hours in front of computer flight simulators before or in the midst of training? I opine that there is a shift in the emphasis away from to plane itself, and toward all of the information that today's indicators provide. Take yourself back to the Tiger Moth, the airspeed is indicated out on the wingstrut, and really is not that accurate anyway, and the tachometer really does not work that well either. I very much doubt that Tiger Moth instruction and operation was filled with all of these numbers!

Just feel the plane, listen to the engine and the airstream, and watch where you are going. During VFR, this should be all you need to fly very well! During my Caravan type training, I had a training pilot who was fixated on numbers. I got frustrated enough with this that I covered the airspeed indicator, and flew a very presentable (and totally safe) circuit. If you need to look at the airspeed this often, you need to improve your skills.

thing

19th Nov 2011, 15:06

All that being said, as I read a lot of the foregoing, I see lots and lots of numbers, but little sense that the pilots are simply flying the plane. Is this a result of pilots spending hours in front of computer flight simulators before or in the midst of training? I opine that there is a shift in the emphasis away from to plane itself, and toward all of the information that today's indicators provide. Take yourself back to the Tiger Moth, the airspeed is indicated out on the wingstrut, and really is not that accurate anyway, and the tachometer really does not work that well either. I very much doubt that Tiger Moth instruction and operation was filled with all of these numbers!

Just feel the plane, listen to the engine and the airstream, and watch where you are going. During VFR, this should be all you need to fly very well! During my Caravan type training, I had a training pilot who was fixated on numbers. I got frustrated enough with this that I covered the airspeed indicator, and flew a very presentable (and totally safe) circuit. If you need to look at the airspeed this often, you need to improve your skills. Who hoo, somebody talking sense. That won't do at all, expect incoming.

GeeWhizz

19th Nov 2011, 15:54

Pace I think throughout this you more or less summed up everything previously. I was not disagreeing with you, merely answering your question...

Why would you be descending at below cruise speed?

fireflybob

19th Nov 2011, 16:36

Why would you be descending at below cruise speed?

Let's say you are flying an a/c that cruises at 300 kts. You have been in the holding pattern at 210 kts and are descending in the hold to FL XX - you then learn that your destination has gone below landing minima and request a diversion to YYYY airport. ATC says "continue descent to FL XX, expect further climb on handover to ZZZ Control and proceed direct to WAYPOINT with no ATC speed restriction"

By the way, the autopilot is u/s!

btw the way another occasion when you might need to do this in a higher performance aircraft is after an engine failure/shutdown enroute - in order to driftdown safely due to high msa (eg over the Alps) you descent with max continuous thrust on the operating engine at the driftdown speed (typically in the order of 230kt) - as you approach level off the nearest suitable airport for diversion is 250 nm away - having cleared the higher terrain you now want to accelerate to a more economical cruising speed (which is faster).

Farfetched? Academic? Whilst I agree we are not teaching all ab initio pilots to go on to greater things, it is fundamental that they should understand and be able to apply basic techniques.

FlyingOfficerKite

19th Nov 2011, 17:12

Academic nonsense really.

There is the method of recovering from a glide descent which is taught in the text books - which is fine for flying training to TEACH AIRCRAFT HANDLING AS AN EXERCISE - as GeeWhizz.

Then there is the 'real World' as IO540 suggests.

The analogy of driftdown in a jet on a Private Flying Forum :yuk:

:}

fireflybob

19th Nov 2011, 17:27

The analogy of driftdown in a jet on a Private Flying Forum

FOK, my apologies if that offends you but don't quite a few private pilots go on to flying jets these days?

Also whenever I hear the term "real world" it means the world as you see it!

FlyingOfficerKite

19th Nov 2011, 17:37

FireFlyBob

No offence taken - or given (I hope?!)

I went on to fly jets.

This is PPrune after all!!!

KR

FOK :)

172_driver

19th Nov 2011, 18:43

Regarding the origional question, whether to set pitch attitude at or slightly above cruise attitude.

The difference is minor, because if you start adding power approx 50 ft before desired altitude (10 % VSI in a C172) the spead will build up quickly. And in all reality, when I glide at 65 KIAS my attitude is approximately cruise attitude so I barely need to change it at all as I level off. The power increase will absorb and neutralise the ROD until I am stabilised at my desired altitude with 0 vertical speed. That's also why I teach (or my FTO rather) full power, the acceleration will go quicker and there will be less need to change pitch attitude as I accelerate.

Works well IMO, many ways to skin a cat though...

PS. Don't forget right rudder, reduce as you accelerate ;)

Cows getting bigger

19th Nov 2011, 19:18

GeeWhizz

19th Nov 2011, 20:00

Didn't realise there was an ex 6,7 or 8 ;)

Big Pistons Forever

19th Nov 2011, 20:05

It may be academic but it is important to understand what is going on. I spend my life flying with pilots who set a power and an attitude and then seem to take another five minutes fiddling with throttle and trim because they haven't previously thought about what is happening. When they finally reach some form of stable state they then realise they are not at the altitude they want. So we then get the pitch up/down scenario together with associated airspeed change and the whol cycle starts over.

Who do I blame? Instructors wh will sign-off an Ex 6-8 in less time than it has taken to pen this message.

I feel your pain :{

The longer I instructed the more time I spent on the foundation exercises. I was looking at my instructor records the other day. The trend is a longer time to solo but a shorter time to the PPL flight test and IMHO a lot better product at the end because of the time and effort I spent on getting the basics right at the beginning. :ok:

Pilot DAR

20th Nov 2011, 01:27

they then realise

There's the problem! We have pilots who are flying this then that then.... The aircraft is best flown in a fluid motion, with smooth transitions from one phase of flight to the next. The less the passenger or observer can detect a change from one phase of flight to the next, the better it was accomplished.

What would we think as the passenger of an experienced driver, who, while approaching a stop in a turning lane, braked a bit while driving straight, then turned a bit with no braking, then signalled, then turned a bit more, then drove straight and braked more and so forth? No, we expect a fluid motion of the aircraft, resulting from appropriate and combined use of the controls.

Sure, when you're getting used to used to a new machine, you may take a few tries at it to demonstrate that fluid motion, but it's what you aspire to.

So it's not "do this, then that.... and so forth". It's transition from this phase of flight to that, while maintaining or changing speed, altitude, configuration as needed, and not exceeding any operating guidelines, or limitations. Other than that, making a recipe out of it is just compicating something which should be very simple and natural.

For all you normally aspirated piston pilots who doubt me, take sticky notes, obscure airspeed indicator, and tachometer, and go flying. You'll find you're more smooth, without the distraction of chasing numbers on instruments!

FlyingStone

20th Nov 2011, 08:17

For all you normally aspirated piston pilots who doubt me, take sticky notes, obscure airspeed indicator, and tachometer, and go flying. You'll find you're more smooth, without the distraction of chasing numbers on instruments!

Sadly, more and more pilots are afraid to fly VFR in 50+ km visibility without an artificial horizon, let alone airspeed indicator.

Pull what

20th Nov 2011, 16:37

Big Pistons Forever

20th Nov 2011, 16:44

If you are descending at the best glide speed you are in danger of ending up on the backside of the drag curve with the OPs instructors method-that might be OK with more powerful aircraft but for light singles near the ground in my opinion thats not best practice

I believe in an early post on another thread you offered your opinion that a post of mine was "unbelievable". Thanks for providing an excellent example of a truly "unbelievable" post. :ok:

Pull what

20th Nov 2011, 16:48

My pleasure

Big Pistons Forever

20th Nov 2011, 17:11

An ex-airforce instructor tought this method of level off from glide descent - it seems this is the technique used in airforce PC7.

Assume glide descent speed is 70kts, cruise is 100kts.

- Initiate level off when 10% ROD of desired altitude.
- Simultaneously select full power and slightly higher than straight & level attitude.
- As airspeed increases, progressively lower the nose to straight & level attitude.
- 5kts before reaching cruise speed of 100kts, select cruise power.
- Hold straight & level attitude
- Trim

The main difference in this technique is that you select slightly higher than s&l attitude initially, then progressively lower to s&l attitude as airspeed reaches 100kts.

His reasoning is, as you are in a glide descent at 70kts, when leveling off if you select s&l (instead of slightly higher), you will still be descending due to the lower airspeed initially.

Any thoughts on this technique?

I think care needs to be taken when adopting SOP's from very different types. Idle power descents in turbine aircraft are quite common, especially when in a "slow down and go down" situation. However, other than the final approach there are very few instances that a gliding descent would be the optimal technique in a piston powered aircraft. I agree with the numerous other posters who questioned why you were in a glide descent to begin with.

If the object is to descend from one altitude to another (particularly in the context of cross country flight) than the most efficient way is to simply leave cruise power on and set a nose down attitude that will give you a 500 foot per min decent rate.

I also agree with PILOT DAR's point about "feeling" the aircraft. How ever what a 6000 hr test pilot can do with unconscious competence is a little harder for a 60 hour brand new PPL :p. But his point is still valid and this is what every low hour pilot should strive for.

Straight and level, climbing, descending and, turning are the foundation exercises of flight training. Key to success in each, is attitude control. So in the context of descending, to fly accurately and smoothly the pilot must know what attitude to go to and then the amount of power set will give him the right performance. Sounds easy but in practice is IMO an often imperfectly learned skill. Too often climbing or descending involves pitching the nose up or down to a random attitude and accepting what ever results.

This is where covering the ASI in flight is a great exercise. Start with the simplest manoever, full power Vy climb. After covering the ASI pitch up to what you think is the right attitude to result in the Vy airspeed, let the aircraft stabilize and trim. Then uncover the ASI, adjust the attitude as necessary and repeat. It is amazing how quickly you will find you can nail the attitude which will result in exactly the Vy airspeed without ever looking at the ASI. Now do the same for the power off glide. Knowing this one is really important in the event of an engine failure, and then start picking random combinations of airspeed and power. Some dedicated practice will produce a situation where flying goes from random control inputs to the "feel" of a truly proficient pilot.

Piper.Classique

23rd Nov 2011, 09:25

Sadly, more and more pilots are afraid to fly VFR in 50+ km visibility without an artificial horizon, let alone airspeed indicator. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/statusicon/user_online.gifSorry to say, but you are right. I flew with a new CPL recently who not only queried the lack of an artificial horizon (in a tube and fabric basic microlight) but couldn't set an attitude without a VSI. Oh, and his stall recovery was interesting, to say the least. Full power, no pitch change, and forgot about his feet so he got a good brisk roll and yaw against the engine. Ho hum. Maybe we should enforce basic training in a glider, followed by J3.

Pilot DAR

23rd Nov 2011, 11:25

I had the delight yesterday to be asked to do the maintenance test flight on a Tiger Moth. It had not flown in 7 years. I was surprised to find that it did have an airspeed indicator in the panel (I still liked the one on the wing strut though - easier to read!). I had to check myself out, as there was no one else available to fly it.

Wanting to be very gentle on the 70 year old engine, but still being weary of it quitting (with no undershoot area), I did high, power on, full sideslipped approaches, so if it quit, I could still make the runway. The airspeed difference between cruise, approach and flare does not seem to be that great in the Moth anyway, so planning descent speeds seemed un-necessary. I just flew it by feel, and it worked out fine.

Trim? Haha!

maxaoa

2nd Dec 2011, 07:56

fireflybob

2nd Dec 2011, 13:19

Think about the reverse, a level off. You climb at say 110kts but want to end up at 140kts. As you approach the height you require (QFE set !!!) you set the att required for initially 110kt cruise (as that is the speed you are at) and then as you accelerate you need to lower the nose (and TRIM) as the att required for the 140kt cruise will normally be lower than a 110kt cruise.

The situation described is essentially the same in reverse. There is no mention as to why they are in a glide descent and the use of full power would indicate to me that they are not paying for the fuel (military training?) however the technique is correct and works on many different types, JP5, Hawk, Grob, PC7, Tucano, CT-4E in my experience !

maxaoa, thankyou for that excellent way of looking at it!

Also, essentially you are doing straight and level part 2 and you can make a distinction as to whether you want to accelerate rapidly to the target speed (when levelling off from a glide in the OP's query) - which case initially select full power - or desiring to accelerate slowly to the target speed, in which case select cruise rpm (with slight correction for fixed pitch as rpm will increase as airspeed increases).

Pace

2nd Dec 2011, 16:04

But what puzzles me with all this is why you would be descending at 110 kts to have to accelerate to 140?
In most cases descending to a level and you are more likely to be at 160kts when you arrive at the target speed ie with cruise power set you will sit there waiting for the speed to bleed off to the 140 kts.
Climbing is different as you will be below cruise speed you will have a higher AOA and more drag as you level and you maybe at full power?

Pace

fireflybob

2nd Dec 2011, 23:18

Agree Pace but OP asked:-

Assume glide descent speed is 70kts, cruise is 100kts.