Descending from FLs

Back_Course

New Member
What is the rule of thumb when deciding how far out you should start descending from the Flight Levels in faster aircraft (assuming you've been cleared)?
 

Derg

Cap, Roci
Staff member
In the real world, we descend fairly close to the altitude restrictions that are on the STAR (arrival). But if there's no arrival, a good rule of thumb is planning to be 30 miles from the field at 10,000 and 250 knots.

Also, if I'm using the vertical speed mode on the autopilot, I'll descent at 2500 ft/min, or I'll use mach .77 to about 24,000, 330 KIAS until about 12,000 and then start bringing the airspeed back to 250 as we approach 10,000.

But usually, (to add even more confusion), you can type your altitude into the FMS and hit the VNAV button and let the FMS handle the descent.

Head spinning enough?
 

MikeD

Administrator
Staff member
[ QUOTE ]
In the real world, we descend fairly close to the altitude restrictions that are on the STAR (arrival). But if there's no arrival, a good rule of thumb is planning to be 30 miles from the field at 10,000 and 250 knots.

Also, if I'm using the vertical speed mode on the autopilot, I'll descent at 2500 ft/min, or I'll use mach .77 to about 24,000, 330 KIAS until about 12,000 and then start bringing the airspeed back to 250 as we approach 10,000.

But usually, (to add even more confusion), you can type your altitude into the FMS and hit the VNAV button and let the FMS handle the descent.

Head spinning enough?


[/ QUOTE ]

VNAV? hehe. I'm still stuck manually flying the penetration track on the HI IAPs.

3D nav would be nice.

MD
 

davetheflyer

New Member
Generally the J41 only goes to the lower flight levels. We generally let ATC handle the descent unless they drop the ball and don't clear us soon enough.

You can cross check ATC by mentally calculating how far the descent will take or you can use the VNAV portion of the FMS. In practice, we normally only use the VNAV when ATC issues a crossing restriction. Otherwise, a guesstimate is usually good enough.
 

MDPilot

Well-Known Member
Try 3 X the altitude to lose, (FL350 down to 2000 is 33) then add 10 miles, or 20 miles if the approach is to be done on your side of the airport. This is no-wind, so you can adjust appropriately for a strong tail or headwind.
For this example, 3 X 33 = 99 plus 10 would be 110 miles before the airport (rounded off). Your airplane may vary in it's profile, but it's a good rule of thumb to start with.
 

NJA_Capt

Well-Known Member
[ QUOTE ]
Try 3 X the altitude to lose...then add 10 miles...
For this example, 3 X 33 = 99 plus 10 would be 110 miles. Your airplane may vary in it's profile, but it's a good rule of thumb to start with.

[/ QUOTE ]

I agree with MDpilot,
3 times the altitude loss works great. Divide the altitude you need to lose my 1000. (10,000' = 10) Then multiply by 3 (10 x 3 = 30 miles). This means you start down 30 miles from your waypoint.

This is only part of the equation though. You just figured out how far to start down, now you need to know how fast (VS) to come down. In something like a King Air, 1500 FPM works well as a ballpark number. When you get into jet speeds in the descent, that figure moves up to 2500-3500 FPM. Both of these examples give you approx a 3 degree descent angle.
At this point you start to consider the deck angle of the plane during the descent. If 3000 FPM is too steep (for pax comfort), then you add the extra 10 miles MDpilot was referring to (30nm + 10 = 40nm @ 1500-2000 FPM).

In a light twin/single doing 120 kts, to lose 3000':
(3000'/1000) x 3 = 9nm. You would need to descend approx 667 FPM to lose 3000' in 9 nm.

300'/nm descent = 3 degrees.
 
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