Best glide speed

[Vee One...Rotate]

Evening all,

A quick question which I found myself (quite randomly) thinking about t'other day.

I understand the concept of an aircraft having a best glide speed i.e. a speed at which the horizontal distance travelled per vertical distance travelled is a maximum. Now, my question.

Let's say I'm at 120 knots and my best glide speed is 80 knots. My engine suddenly fails. Do I:

(a) Convert that excess 40 knots above best glide to altitude. This way I assume best glide speed but at a slightly greater height thereby maximising my gliding distance (and options).

(b) Assume best glide naturally i.e. as speed bleeds (assuming staright and level).

I would have said (a) as it appears the best way of maximising gliding distance. However, this is a theory I thought up while just musing over this on the way to work today and wanted to ask: what is the official line? At least with regards to light aircraft flying (though I presume the answer is equally applicable to larger types)???

Cheers for any input,

V1R

[Gulf Julliet Papa]

Surly the best answer would be to try and do a mix of both....

When your engine fails at say 2000ft, apply back pressure so you stay at 2000ft, but your speed decreases. Once your plane hits 80kts reduce the back pressure and put it in a steady glide.

Ive been taught both methods, but thats the way i passed on!

[chrisN]

For pure gliders, its method a (except when in ground effect, maybe) - the sooner you reduce drag the more energy you conserve. I should have thought the same applies to most if not all aircraft once it has turned into a glider.

Chris N.

[Sunfish]

Standard textbook reply is to hold the nose up and trade speed for hieght until you his best glide speed then lower nose and trim. Hieght is better because you dont know what direction you ultimately need to glide in.

[trevelyan]

Yeh with sunfish - if the donk ever dies and your above your best glide speed always convert excess speed into height. Spesh if you were at 120kts and say your target glide was 76kts. Would gain you some very usefull height.

More time to decide what to do and where to do it..



:ok:

[DubTrub]

V1R: Yes, trade speed for height (therfore time).

Just for practice, and a bit of education, try this: (assuming aeroplane)

From cruise speed, gradually reduce power (add carb heat if necc.) and increase nose-up trim...slowly...until you are at idle power, full nose-up trim.

Take your hands and feet off.
Let the aircraft descend naturally.
Notice the speed?
And the rate of descent?
And the rate of turn?

Give it a go, it can be an education!

[It's quite a non-event actually, and perfectly safe if the aeroplane is correctly rigged, but the numbers can sometimes be educational]

For aeroplane pilots more experienced, then try applying full power (but watch for the pitch-up, you need to push to counter-act). This has caught many out in the go-around. [Guess why Cessna stopped doing 40 flap aeroplanes?]

[Cabotage Kid]

Yup! The only answer is to exchange speed for height. You get more range and more time.

Personally speaking, the last thing I would want to be doing is analysing the situation.

[Whipping Boy's SATCO]

On the other hand, if you are IMC at the time (God forbid!) it may be better to keep the speed (ie energy) such that when you break cloud you can then trade speed for maintaining hgt.

[englishal]

never really thought about it before, just always been taught to convert the excess speed to height straight away, but chrisN's reply makes total sense......

At higher speed drag will be higher, and hence wasting energy. If you yank back, say climb a few hundred feet as the speed (and hence drag) reduces, you'll convert more energy into "glide" energy, and hence go further.....

Cheers;)

[dublinpilot]

Well, there is more to it than just conserving energy. You need to find somewhere to land too. A climb generally reduces ground visability. Therefore holding altitude until speed has reduced to glide speed, may give you more useable time to look out for a good landing site. Just a thought.

[IO540]

Statistically, a suitable field can be equally near or far. So I would fly level and look for a field.

[Vee One...Rotate]

Thanks for the feedback everybody. After having gone on a number of trial flights I have my first lesson of the PPL this coming Wednesday. Hope the weather's decent...

Cheers,

V1R

:)

[david viewing]

Shurely not!

The 'pull up' manoevre must be less efficient (wing loading and therefore drag) than allowing the speed to decay to best glide speed. I agree that's what is taught, but the gain in height will be at the cost of overall glide time, albiet a marginal difference.

On the other hand, you might just see that crucial field from a higher elevation. Whether you can glide to it is another matter...

[dudduddud]

I think maintaining straight and level is the best way for two reasons. Firstly a sharper decay in speed will reduce the overall glide range and secondly, it gives you a head-start on the engine-falure checklist.

As the airspeed in the zoom-climb plane washes out quickly (increased drag etc...), the straight-level plane will inch out, and depending on the type, you may end up with a greater glide range. In any event, I would say the difference are nominal.
Once you have decided on a course of action, stick with it.

Secondly, I think the first thing you should do after an engine failure is try and get it started again and as far as i can tell, waiting till after a zoom-climb to start those checklists doesn't help the resuscitation odds (you weren't going to try and diagnose problem in a zoom-climb, were you?).
Also:
While a 'zoom-climb-to-best-glide' pushover isnt rocket science, it is more difficult than a straight-level drift into glide attitude thusly easier to stuff up a bit, causing a loss of range.

[FlyingForFun]

I think the first thing you should do after an engine failure is try and get it started againSorry, Dud, but I beg to differ.

The first thing you should do after an engine failure is set the aircraft up for best glide speed, using whichever method you feel most comfortable with (since it seems the consensus is that there's not much difference). The second thing you should do is pick your field. The third thing you should do is plan your descent towards your field.

Getting the engine started is the fourth thing you should do. If the engine starts, great, you're on your way again. But if it doesn't, at least you've already got yourself set up for a, hopefully, successful forced landing.

On the other hand, if you try to restart the engine first and it starts, great. But if it doesn't restart, you may well have gone out of gliding range of your only good forced-landing option.

FFF
--------------

[Tinstaafl]

I go for maintain altitude while decelerating to Vg. It's less workload & allows checks/field search to be done while it's happening. A zoom climb requires somewhat more of the pilot's attention to control things. There's an arguable case for a 'g' induced energy loss caused by zooming however at the 'g' levels most pilots use it would be negligible, even if it's discernable so not really a factor in choosing whether or not to reduce speed gradually.

There is a circumstance where I would choose a height gain: When visibility range is reduced as a result of low level flight. In this case the zoom to gain height has a pay off in the vastly improved observation range. Doesn't apply once more than a few hundred feet high.

[Cabotage Kid]

Hmm, I'm not sure. Exchanging speed for height gives you more options simply because you are higher. Maintaining alt will probably be better if you are already heading in the general direction of a good landing spot. However, it would probably take valuable time to confirm that is the case.

I'm not sure of the idea of a zoom climb, not sure what one is, but that sounds like you just yank back on the stick to use inertia as well as speed to carry you up. Would that be a better way than just say pointing the nose 10deg up?

I really think such moments are not a time for analysing. I would always endeavour to make the first instinctive action the one that, on balance, would probably present me with more options.

[Paul Wilson]

I was taught to PANIC

Persperate - get this out of the way quickly

Aviate - set for best glide speed

Navigate - choose landing site

Investigate - try to get engine started again

Communicate - Declare Mayday

[Cabotage Kid]

:D

I wonder how many of those big red PANIC buttons Transair actually sold.

[Tinstaafl]

CK, a zoom climb is a climb where the energy for the climb is obtained by trading speed for height. It doesn't have to be steep but it does take more attention of the pilot to do compared with maintaining S&L while speed reduces. The a/c can be kept in or near trim, reducing the workload. There are fewer attitude changes needed and the a/c ends up at the glide attitude & speed with relatively few comnplications.

Exchanging speed for height has the added complication that the a/c has to be deliberately pitched NU (when it's trying to pitch ND), held against this while speed is converted to altitude, then pitched ND to achieve an attitude for Vg before stalling.

WRT to more options with height. This isn't any different to options with speed. It's all energy. Climbing only converts one to the other. If you're at more than, say 500' or 1000', there isn't really a useable difference in areas you can see & glide to. Unless there's an obstacle such as a ridge or mountain of course but then that's analogous to low level flight where a gain in height dramatically improves observational area.

[Cabotage Kid]

Hi Tinstaafl,

I daren't speak for everybody but personally I would find it harder maintaining alt with the speed bleeding off than sticking it 10 deg nose up until Vmd.

Regarding the options, again I beg to differ. If you are higher you are higher - even 5' is the difference between clearing a hedge and not. Unless you have the sea or a mountain range behind you or you are completely alert as to the best place to put down before it goes "phut" (virtually never in my case) then, in my book higher is better.

I'm doing PFLs next week on my CPL so I'm keen to see how that is taught.

I guess we can beat this subject to death without getting anywhere. I hope that we haven't muddied the waters too much for Vee One!

[Vee One...Rotate]

No, not muddied. Very useful discussion - it means I (and others) can make up their mind what is best in a certain situation with more fact/informed opinion to go on...can't be bad.

Had my second lesson of the PPL on Saturday and I'm loving it so far :ok:

V1R

[Wrong Stuff]

Whipping Boy's SATCO said:
On the other hand, if you are IMC at the time (God forbid!) it may be better to keep the speed (ie energy) such that when you break cloud you can then trade speed for maintaining hgt.
That's a very good point.

The only times I can think of when it might not be better to keep the speed up are: when you're unsure whether you really will be breaking cloud before you hit something,
if you're over water or inhospitable terrain and an extended glide could take you clear; or
when there's an advantage to staying up as long as possible, eg if there's a chance of a restart given enough time.

[Tinstaafl]

CK, why not have a go at doing it each way? Try the zoom method, then try the S&L method. Do both ways while also trying to do your checks and look for a field.

I know which method I - and my students - find easier.

WRT to your comment about additional height & clearing obstacles etc, I think you've misunderstood something: using speed to maintain altitude until at Vg, or changing speed into height first then maintaining Vg is largely irrelevent to glide range. It's all about potential & kinetic energy. Zooming & diving changes one to the other. If anything, the additional 'g' required to initiate the climb has an adverse effect in terms of wasting some of that precious energy because it adds drag. Mind you, at the typical 'g' levels used to initiate the climb I think the difference is negligible, making the decision to zoom or not being based on practicality ie does it do anything for visibility?

[Cabotage Kid]

I'm pretty sure I understand ;)

My point is that if you maintain height and let the speed bleed off, then unless you are heading in the right direction at the moment of failure and taking into account thinking time, then your effective range to the field to the side or behind you that you can land in is reduce by virtue of the fact that you have maintained your height and travelled futher away or maintained your distance.

[robtherower]

An intersting topic - I had a purely gliding background before taking up aviation as a career, so my personal preference is the pull up to wash off speed. In a glider I would use a 2-2.5G pullup and a 0G pushover, obviously that's a bit dramatic in a light a/c for passenger comfort and the maps etc bound to be strewn around the cockpit.

My reasoning is this - a light a/c drag curve is quite steep, that is to say that if you are not very very close to your best glide speed the aircraft will be performing poorly. For a normal single engine piston you will probably be a good 40-50kts over your best glide in the cruise which will absolubtly destroy your glide ratio.

The pullup method therefore gives the benefit of extra height and time over the straight and level method. Of course like many things in aviation this is just my preference and there is not a single correct and accepted method.

[Send Clowns]

The only time I have been taught to trade speed for height after engine failure is at low level. Then the speed needs to be back to glide as quickly as possible to stabilise the approach for judging landing position. However most civvies should not be flying at low level (stand fast commercial helicopter pilots on the wire inspection or police flights) unless forced down by weather, and if it really isn't your day and you lose the engine during a weather diversion then you shouldn't climb as you may enter cloud.

I would therefore agree with those here that staying approximately level until best-glide speed is reached is what I have been taught and told to teach.