What is a typical glide angle for a modern passenger jet with engines idling, with full flaps and with the gear down?
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1$\begingroup$ Related: How does the Boeing 787 Dreamliner's glide ratio compare to other airliners? $\endgroup$– minsFeb 28, 2017 at 18:40
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$\begingroup$ Slightly related Why is a 3 degree descent angle used? $\endgroup$– Notts90 supports MonicaFeb 28, 2017 at 19:12
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2 Answers
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It must be in the region between 1/5 (11.3°) and 1/10 (5.7°), and closer to the shallower angle. I guess you want the value at 1.3 times stall speed - without a speed an answer would be impossible.
Once the glide ratio drops below 1/5, the energy dissipation is too quick to allow rotation for a soft touchdown with idle thrust. On the other hand, a glide ratio much in excess of 1/10 will make the aircraft float along the runway in ground effect, which makes the precise selection of the touchdown point (at the correct touchdown speed) hard. Spoilers help to adjust the glide path angle to steeper values, so the aircraft with flaps and gear fully deployed but the spoilers in should have a glide angle around 5° to 7°.
answered Feb 28, 2017 at 18:53
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2$\begingroup$ The requirement to go around with OEI does not permit much worse than the 1/10. E.g. A320 (which IIRC can use full flaps even with OEI) has MLW 66 t and single engine thrust 98 kN (the least powerful variant). Out of that, 21 kN is needed to achieve 500 ft/min climb rate at 150 knots and what remains gives weight/thrust ~8.6. $\endgroup$ Feb 28, 2017 at 23:45
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Depends on the requirements of the approach. A typical PAPI or ILS glidepath is 3.5°. But some non precision as well as precision approaches are steeper. The LOC/DME-E for KASE has a steep final approach angle of 6.59°.
Peter's assessment of glide angle is good as well, though one might be able to fly a little steeper with judicious use of power when entering the round out to arrest the descent.
answered Feb 28, 2017 at 19:44
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$\begingroup$ Typical ILS/PAPI is 3.0, not 3.5. That "E" approach is a circling approach, not a straight-in. The question specifies Full flaps, gear, and IDLE thrust, and in those conditions you won't be able to hold a 3 degree (nor even 3.5 degree) glideslope -- you'll bleed airspeed pretty quick with that much drag. In a 737, idle/gear/flaps 40 probably gives about 5 or 6 degrees, depending on winds & weight -- plenty of capability to catch a glideslope from above. $\endgroup$– Ralph JMar 1, 2017 at 17:24