Some suggestions for a decent descend?

[Alitalia 0]

Hi!

I have a problem when start descend with my 737... i'm at cruise level 210 and the ATC tell me "...start descend to FL 150 and reduce speed 250"; i select 250 speed and 15000 then press LVL/CGH mode with speedbrakes deployed, but the descend start slowly with 100, 200 max feet/min; whern i approach to 250 knots, the ATC tell me "..please, expedite your descend !!!"; then i set 2500feet/minutes descend, but even with full drag, my speed up to 300kts (sic!).

Please, help me, before the air brakes catch fire :-))

Thanks

Patrizio

PS: i tried VNAV DES NOW without success...

[Jgid 0]

Hi!

I have a problem when start descend with my 737... i'm at cruise level 210 and the ATC tell me "...start descend to FL 150 and reduce speed 250"; i select 250 speed and 15000 then press LVL/CGH mode with speedbrakes deployed, but the descend start slowly with 100, 200 max feet/min; whern i approach to 250 knots, the ATC tell me "..please, expedite your descend !!!"; then i set 2500feet/minutes descend, but even with full drag, my speed up to 300kts (sic!).

Please, help me, before the air brakes catch fire :-))

Thanks

Patrizio

PS: i tried VNAV DES NOW without success...

When flying, you can either descend or slow down, but not both at the same time. In this case, choosing LVL/CGH would have done what you wanted, except, it will take much longer that what would be needed.

 

Level change is a pitch to maintain speed mode. It works in combination with the auto throttle to reach the selected altitude at the desired speed as quickly as possible. In the case of large descents, such as this one, flight level change will reduce the throttles to idle, and pitch the nose to reduce speed to 250kts. The reason that you are only getting a V/S of 100-200 is because that is the most the aircraft can pitch down and still reduce speed to the 250kts. Once the aircraft is slowed to the selected speed, the descent rate can increase, because it is easier to maintain a speed while descending.

 

If you are using default ATC, it is possible that you are simply not being given enough time to descend to that altitude with that speed. For the most part, if VNAV is left on, and you begin your descent at the top of descent, you should be able to maintain approach altitude restrictions, etc. If you must descend after the top of descent as per ATC instruction, try using LVL/CHG with a higher seed selected. This may help increase the rate of descent. VNAV descent paths are just like LVL change with the pitch to maintain speed function. You could try descending with a V/S that will descend faster, also increasing speed, but then reducing that speed once reaching the selected altitude.

 

The below YouTube video is by another AVSIM member, and explains planning descents in greater detail.

[speedbird034 14]

I believe you are asking how to get to 250kts at 15000 from FL210...

 

Typically (at least in real world), ATC will give you one of two instructions (in the case of there being no speed/altitude constraints in the arrival), "Slow to 250kts, then descend and maintain 15000."  OR  "Descend to 15000 and maintain 250kts".  These are two very different things.  The first is quite specific, you must slow down then descend, the second is descend to 15000 and be at 250kts when you get there.

 

At FL210, you probably shouldn't be at more than 290kts...but even if you are at, say, 310kts, the same is the case.  Speed intervene by selecting 250kts, slow to 250kts then select level change.  In this way, you should be able to hold 250kts and descent at a reasonable rate.  Of course, there are some things we don't know from your post, e.g. how far from your present position do you have to be at 15000ft...but if you remember the 3 miles for every 1000 ft of altitude, you are going down 6000ft...multiply by 3 miles and you should plan to be at 15000ft at 18 miles from the point you start the descent.  It won't take a long time to slow from 290 or 310 to 250, just simply select 250kts in the speed window and the a/t will throttle back to give you the needed drag to slow you down.

 

If you don't have to slow down first, the same technique holds, except you would speed intervene, say, 270kts, so that the aircraft will maintain 270kts in the descent. then at 1000ft above your target, select 250kts and when you level out, you shouild hit your target of 250kts once you level out at 15000ft.

 

You could use VNAV; however, you'd have to select your next way point at/below 15000ft and speed select 250kts in the MCDU then execute it.  It's a lot of button pushing that probably isn't necessary...unless you are given a constraint that says you must be at a specific fix at 250kts/15000ft.  THEN it is usually better to put it in the box and VNAV down to it.  From 290 or 310kts you should not have to employ speed brake to slow you down...but if you do, just remember you will increase your rate of descent at the same time, so beware that if you are trying to hit a specific target, stay off the speed brakes unless you have to do so.

 

 

 

Hope this answer helps.

Dave Lamb

 

FAA Licensed Aircraft Dispatcher

KIND

[Cristi_Neagu 134]

 

 

The reason that you are only getting a V/S of 100-200 is because that is the most the aircraft can pitch down and still reduce speed to the 250kts

 

Actually, I believe LVL CGH will only pitch up to -1000fpm while descending and slowing down. It does that so it can comply with ATC descent instructions who expect you to have at least -1000fpm in descents.

[Alitalia 0]

Thanks guys for your reply!

[kevinh 409]

Actually, I believe LVL CGH will only pitch up to -1000fpm while descending and slowing down. It does that so it can comply with ATC descent instructions who expect you to have at least -1000fpm in descents.

LVL CHG will pitch up to maintain the selected speed. It does not maintain a minimum of -1000 fpm. If LVL CHG does not give sufficient vertical speed to satisfy ATC you can always use speedbrake to increase rate of descent.

 

The autopilot isn't designed to satisfy ATC demands. Especially as these can vary around the world. It's up to the pilot to comply with those.