Estimating dead-stick glide performance.
I've been thinking about this for a while and just want to confirm my suspicions.
Flying B737-400.
Flaps Up.
210 knots.
Rate of descent = 1000 fpm.
Flight-Idle thrust fuel flow = 350 kg/h per engine.
I'm trying to estimate how much the idle thrust is contributing to the glide performance and how much the glide path angle will increase with two dead engines.
The other day while in this configuration (the first one mentioned above!) I pushed the thrust leavers forward until the fuel flow doubled to 700 kg/h per engine and noted that the rate of descent reduced from 1000 fpm to 500 fpm.
I'm just wondering. Does that mean that the idle thrust of 350 kg/h is contributing 500 fpm in this configuration and speed?
If I lose both engines, can I expect the rate of descent to be 1000 + 500 = 1500 fpm while on approach at 210 knots? Doesn't that mean then that if the rate of descent increases by 50%, that I have to increase my altitude gates by 50%.
So at the start of final approach where I would normally be at 3000 ft, I need to be at 4500 ft. Or another way of looking at it, where as I normally allow around 3 nm per 1000 ft, I now have to allow around 2 nm per 1000 ft.
Does that sound about right?
Also the fact that at the higher levels, idle thrust fuel flow is only about 150 kg per hour or less, does that mean that loss of thrust on both engines does not have such an affect on glide performance at higher levels than at lower levels where the idle thrust fuel flows are around twice this?
By the way, I've done some dead stick landings in the simulator without the APU supplying electrical power for the electric hydraulic pumps. With only the windmilling engines supplying hydraulic power, you do NOT want to get any slower than about 190 kt with any more than Flap 5 for two reasons. Firstly, any slower and the engines are no longer turning over fast enough to run the engine driven pumps and you start getting that manual reversion feeling through the control wheel. Secondly, with any more flaps, the rate of descent becomes excessive and you will be unable to arrest the rate of descent in the flare. And if you try to arrest the rate of descent by flaring a little higher than normal, all you do is bleed off the airspeed and encounter the stick shaker and stall it while the high rate of descent remains.
Anyway can someone please confirm or deny my estimations of the idle thrust contribution to glide performance?
Thanks.