Originally Posted by
hans brinker
EPR (engine pressure ratio) is the main thrust indication in some jet engines. It is the ratio of pressure in front of the engine to behind the engine (simplified). All the ones I have flown had a number below 1 at idle descent, so no forward thrust. The Citation Bravo would do 5000fpm VMO at idle without speed brakes)*. The 320NEO when you go to idle in the flare as a pronounced nose down from the drag, very much doubt it has forward thrust even at approach idle, never mind flight idle. Sure, it's not like a turbo prop with the props at full fine, but it's not forward thrust. A320 I would reduce to idle around 30' +/-20' depending on gust/weight/approach angle/runway slope. New to the 737, but they teach start reducing below 30', idle just before touchdown (I think that is too low, but new to the airplane....)
)* best I have done was a 32 minute flight, with 6 minutes at FL370 for engine trend monitoring after MX.
You really don't need to tell me what EPR is....... Yes, below 1.00 then the intake drag > gross thrust out the back but that is a factor of TAS/MN & i was talking about on final approach at relatively low speeds, that is after all the title of the thread "Landing". Yes, i'm sure there will be net drag with very high bypass ratio's, such as the LEAP engine on the NEO 10-11:1 BPR but i was flying the CF34 a more conservative 6.4:1. (and it was an N1 only engine, I'd much prefer EPR too.) I don't think the earlier fan engines (RB211 etc at 4-5:1) went below 1.00, not regularly anyway. Yes, I generalised too much with the jets. However a pure prop will always produce net drag at idle, i have vague recollections of the instructor setting "zero thrust" on the "failed" engine during training. I was glad to get away from MEP's after qualifying tbh, awful things!
And sure a bizjet will descend fast if you push it down and go at Mmo, but it's not a very efficient, elegant or comfortable way of doing things is it?