Originally Posted by
Private jet
Huh?? No mate. Piston props produce more drag than thrust at idle, so net drag. That's why they descend like a brick when you close the throttles. Jet's still produce forward thrust at idle (about 4-5% ish of maximum) so they need a shallower descent and need a lot of runway if you are not on Vref. It was a long time ago but i remember powering light twin all the way down to just above the ground; But bizjets (Challenger) you need to start creeping the thrust back at 30-50ft.
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.