I have done this subject before - but I cannot find it on my local disks or in PPrune - so here we go again. Usual caveat from me - granny and her eggs. I will keep it simple - interested parties can try it out for themselves.
The other caveat here is that all of this is the result of simulator work. Simulators are good, but they sometimes skate over some of the details that exist outside of the "normal" flight envelope. That said, the consistency is high on several different sims - so it should be very close to the real world. However all that follows is only a "best guess" as to what will happen in the real event. With that in mind there is lots of leeway ...
Before I start let me assure Mr Pax and 126.9 that a B777 glides as good as anything out there. In fact a good deal of what follows is based on the B777, but most of it is the 767. That said, all modern Jet equipment has similar attributes and this technique is very transferable across types.
OK - this is going to a long one folks ( I have 3 hours to kill before off to XXXX (agghhh)) - fast forward now to skip the intro....
So that I can draw in some of our non-jet and lesser qualified readers - let me do a little mass brief :
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Gliding :
All aircraft glide - even helicopters (although not well I must admit). However all Fixed Wing aircraft have a potential glide performance and some are better than others. As a general rule, the more fuel efficient the aircraft the better it will glide. This is simple math since gliding consumes energy just as powered flight does. In the gliding case the energy comes from the potential (Altitude * Mass) reserves rather than from engines and fuel. At the end of the day it is a question of drag. Drag consumes that potential energy, just as it consumes fuel under power. Heavier aircraft (with more Mass) tend to glide better (further) than light ones. This is a quicksand topic and I intend to skate quickly over it - the answer depends on the wind affecting the glide. Let's take wind out of the equation today - this is not a PhD course.
As people have rightly stated earlier, the aircraft is normally fully equipped to glide. Just because the engines stop, does not mean that it falls out of the sky, far from it. The normal descent procedure for a Jet aircraft is to throttle back to idle thrust and use all that potential energy to get it the last 100 miles or so to destination using minimum fuel. Indeed the whole skill is to reduce the enormous amounts of energy that are resident in a 200+ ton Jet at 37000 feet doing 500+ kts to zero by the end of the target runway.
The normal method is simply to take the height (in thousands), multiply it by 3 and add a factor for speed to start a throttles closed descent. Normally this factor is about 15-20 miles. So if you start the descent at 35000 you get (35*3) + 15 = 120 miles. Without the complications of wind, this will allow you to GLIDE (throttles closed) to with 5 miles of your landing. Trying to finesse the problem closer than this is usually a bust since you do need some engine power on the finals to allow you a level of control - but once again this is too much detail at this point.
OK, so assuming a normal Jet at normal altitudes. If all the motors stop more than 100+ miles from an airport - you are looking at a real problem. However if you have a half decent runway (how long is an issue here - but anything tarmac or concrete is better than dirt or water..) nearer than that then you have every chance of a successful glide approach. The trick is knowing how to do it.
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One of the larger problems that is often ignored on this topic (not that this is often discussed or taught) is that of pressurisation. Most twins allow bleed air from the APU to be used only below 14000ft. Most quads don't even have an APU that can be started in the air. So pressurisation will be an issue. Obviously it will depend on the age of the aircraft, condition of door seals etc, but there will be a significant DeltaP reduction once the engines stop. This may be a limiting factor in determining ultimate range - and it is not a trivial one. There may well be an imperative to get the airframe to 15000ft or so PDQ and this will restrict the ultimate gliding rage. However let's assume that is not the issue.
Let us now progress to a no wind day at 10000ft within 30 nm of a suitable airport. It is your day !!! From now on in I will assume we are in a 767 / 777 even a Jurassic 744. (Although I will agree that the situation is less likely in the latter - unless we are talking fuel contamination or similar)
If you have an ILS on your intended runway - life is good. Otherwise work for a 3% / 3 nm per 1000 slot somewhere on finals. You have plenty of leeway above this - but not a lot below it. Work on the profile accordingly.
Configuration is clean / Vref + 80. Use airbrake to adjust. The RAT is out and the APU (if you have one) is on line and all systems are working. Try to adjust now to a 3.5% slope - 400ft / mile and once you achieve that, drop the flap to 5 and reduce to the 5 flap speed. On the ILS this 3.5% looks like 1 dot high. Stay with that using airbrake to control speed and elevators to control slope. Stay with this profile to 400 ft. There is lots of flex here. Nominal zero wind should have you at about 1/3 to 1/2 brake. Ensure that AutoBrakes are OFF.
At 400 ft dump the gear and all the flap. Aim for the threshold and use the extra speed to kill the rate of descent. At this point it is the ROD that will kill you. Get back to a nominal <1000 fpm using the excess speed.
Touch down where you can - the sooner the better. Deploy the speed brakes and wait. All those lovely V squared thingies are coming to your aid here. When and if you get to the last 2000ft then start applying brakes.
There you go - it's almost easy and very repeatable. Give it a try (in the Sim!). Any feedback will be well received.
MG
[ 25 August 2001: Message edited by: MasterGreen ]