Don S
Lv 5
Don S asked in Cars & TransportationAircraft · 6 years ago

# How far from it's destination will the pilot of an airliner commence its descent?

Lets a jumbo is at a cruising altitude of 32K feet flying at 450 knots. How far from it destination will the pilot commence his descent assuming that he/she is not under ATC control. Does the aircraft descend in increments or does it go into a lengthy steady descend? As the aircraft descends on autopilot of course, does the autopilot adjust the air speed? or does the pilot have to adjust it manually.

Relevance
• 6 years ago

Start descent at distance of "THREE TIMES THE ALTITUDE" -

That is the standard figure used by pilots for most airplanes -

Engines at idle thrust -

Example -

You are at FL370 (37,000 ft) = start descent 3 x 37 = 111 NM from destination -

That is assuming "NO wind" -

You have to compensate somewhat for headwinds or tailwinds -

Means start descent early - or later -

Speed used for most airplanes is...

(1) Cruise Mach number - (2) then 320 KIAS to FL100 - (3) then 250 KIAS -

Either "straight-in" on the ILS (start 10-15 NM earlier) -

Or "circle to land" for landing (delay top of descent 10-15 NM closer)

Ballpark rate of descent 2,500 FPM -

Adjusting descent is "fly faster (340 KIAS) if getting too high on profile" -

If getting too low on profile - "stretch descent - slow down to 300 KIAS" -

Autopilot is set on "speed select" (maintain KIAS through descent) -

For the example, would select IAS to 320, then 250 KIAS at FL100 -

Be at FL100 at distance 30 NM before touchdown point @ 250 KIAS -

Start configure flaps 20 NM from touchdown (reduce below 250 KIAS) -

Source(s): Retired airline pilot - I used above figures for 707 - 727 - 747 - DC8 -
• Ianab
Lv 7
6 years ago

Skipper has given the actual numbers. As for the other questions. You would try and descend in a steady glide slope with reduced power as thats the most efficient way (time and fuel wise)

I only fly as a passenger, but I notice if we are descending into a smaller airport with little traffic ( Rarotonga for example) the descent is very steady, with little change in throttle or angle until the flaps and gear deploy a couple of minutes out. This is controlled airspece, but into an airport where you may be the only aircraft within 200 miles. The pilot can prettty much line up the landing from ~100 miles out.

Now coming into a busier airport (Sydney for example) with other traffic you can tell the pilot is working harder and descents are in stages, with more turns to get into traffic patterns etc.

• Don S
Lv 5
6 years ago

As you may notice from above I already selected a best answer.