Alt Intervention and Speed

[Bman2006 7]

 

Hey all-

 

I have found that I am constantly fast coming down from T/D. Some times to the point where the FMS pretty much gives up on attaining speed/altitude restrictions. I deploy the spoilers and drop back to idle but still have some trouble getting slow enough.

 

Should I do an altitude intervention prior to T/D or drop back to idle and do some a speed hold? What do you guys recommend to stay on speed and target during descent. Do get me wrong, I am close but I still end up using speed and V/S to get things under control.

 

Second question - what is the relationship between ALT intervention button and FMC? At what rate will alt inv descend?

 

Thanks-

[Spin737 102]

1. Make sure you're at idle. Idle changes as you descend, and some have reported that a higher idle stays locked in when in a lower N1 should be active. I'll do an ALT INT/Descend Now if I'm above FL350 before the T/D at 4nm per 1000ft above FL350.

 

2. It'll descend at 1000fpm until it crosses the VNAV PATH.

[fakeflyer737 992]

Fill out the Descent Forcast page? That makes a HUGE difference.

[Prisco 9]

Try to change the costindex. If you you set the CI higher the descentrate raise, but the aircraft doesn't accelerate.

 

Joachim

[Spin737 102]

Try to change the costindex. If you you set the CI higher the descentrate raise, but the aircraft doesn't accelerate.

 

Joachim

How would the airplane descend at a higher descent rate without increasing airspeed?

[Rickie Welsh 27]

Hello, Look at the predicted speed in the FMC for descent w/points and using speed intervention set speed to that figure then select Flight level change. This method is preferred by RW Pilots that I know and keeps the speed almost constant. richard welsh. PS Forgot to mention amend flight level also.

[Bman2006 7]

Lots of good information here guys - thakns for that.

Follow up questions:

 

1) I understand what cost index does, but I never really know where to set it. I assume the higher the cost index value, the more economical the plane with behave in terms of climb and cruse thrust rates? I always set it at 50 since I never really know what will happen if I set it lower or higher.

 

2) FL CH - I will have to read up on it. My understanding is that the plane will descend at a constant speed (either what the plane is currently flying) or as you say Richard, based on what ever speed intervention is set. Question - won't the plane under vnav want to descend and cross the waypoint and speed restriction already set in the FMC? Why go through speed intervention and FL CH if the plane is going to target itself to cross the waypoints at FMS specified speed and altitudes ?

I assume by setting the speed intervention then decending, the plane will slow down on the front end of the descent versus trying to slow down when the plane ie already on a full descent path?

 

Benjamin

[Prisco 9]

you can change the Ci during flight, you will see that the speeds changes.on earlier days i flew with a ci 20 and the aircraft reduced the speed on descent to 260 knots.....i hink ist very economical. If you set the ci to 55 the speed on descent

accelarate to 305 knots and the descentrate goes to 2300 - 2600 f/m.

its very easy, try it, there ist no risk ;-)

[JazzPilot 0]

On my recent jumpseat on the 738 the CPT was PF and the F/O was PNF, the we were at FL380 and were cleared to FL280, around 10 miles prior to T/D the CPT used V/S -750, then after a few seconds ~ -2400 at the V/S, and after a few seconds he hit VNAV, the speed target when he hit V/S was .77 mach, which was our CRZ SPD, then the aircraft went back to -1000 ft as we were below the descent path, once we captured the path it started to descent faster, no spoilers were needed til we were close to 10.000 ft which was the time where we slew the aircraft down to the restricted speed, that was the point we used the spoilers.

 

That's the way I do it, different real life techniques helped me a lot, made me a better sim pilot and hopefully a real airline pilot soon. That's only a pointer I can give you here, not usre if it will help you in the future.

[Spin737 102]

Lots of good information here guys - thakns for that.

Follow up questions:

 

1) I understand what cost index does, but I never really know where to set it. I assume the higher the cost index value, the more economical the plane with behave in terms of climb and cruse thrust rates? I always set it at 50 since I never really know what will happen if I set it lower or higher.

 

2) FL CH - I will have to read up on it. My understanding is that the plane will descend at a constant speed (either what the plane is currently flying) or as you say Richard, based on what ever speed intervention is set. Question - won't the plane under vnav want to descend and cross the waypoint and speed restriction already set in the FMC? Why go through speed intervention and FL CH if the plane is going to target itself to cross the waypoints at FMS specified speed and altitudes ?

I assume by setting the speed intervention then decending, the plane will slow down on the front end of the descent versus trying to slow down when the plane ie already on a full descent path?

 

Benjamin

The lower the CI, the more economical the plane flies.

 

I think my airline uses CI15 unless we're doing ETOPS, then it's 45. I've seen 60 if we're fighting a strong headwind.

 

I guess marketing has decided that being the on-time airline is worth more than the higher fuel costs.

[rsvit 16]

If you are above VNAV path you can also enter new higher speed on PERF DES page, FMC will recalculate path and you can even get below path after that.

[Spin737 102]

around 10 miles prior to T/D the CPT used V/S -750, then after a few seconds ~ -2400 at the V/S, and after a few seconds he hit VNAV, the speed target when he hit V/S was .77 mach, which was our CRZ SPD, then the aircraft went back to -1000 ft as we were below the descent path, once we captured the path it started to descent faster, no spoilers were needed til we were close to 10.000 ft

That wouldn't really help you descend to stay on path, but it would make it a smoother transition. I do sometime similar at TOC. But that's just for the PAX.

[richardnobes14 13]

Use Speed Intervene or FLCH during the descent and set it to between 310 and 320kts, works best below FL200, let this run for a while as it is takes you below your VNAV PTH, wait until you are about 1,000ft below and then wind the speed back to your desired speed (usually 240kts if below 10,000). This gives you plenty of extra room to slow down without having to use the speedbrakes or initiate an early flap extension.

 

Be sure to monitor any terrain and adhere to MSA's and ATC instructions though.

[Voj 0]

you can change the Ci during flight, you will see that the speeds changes.on earlier days i flew with a ci 20 and the aircraft reduced the speed on descent to 260 knots.....i hink ist very economical. If you set the ci to 55 the speed on descent

accelarate to 305 knots and the descentrate goes to 2300 - 2600 f/m.

its very easy, try it, there ist no risk ;-)

 

Doesn't the CI only affect the climb/cruise phases ?

[Spin737 102]

 

 

Doesn't the CI only affect the climb/cruise phases ?

Nope - all three. Of course you can just override the CI and put in whatever you want on the the climb, cruise, or descent pages.