Spoilers & 737NG behaviour

[mauriceb]

This likely has nothing to do with PM (although it might), but I'm just wondering if anyone flying any 737 flight models has any success with speed brakes when it comes to slowing down the airplane.

I use the PDMG 737-800 flight model (no panels) and I cannot see any evidence that the spoilers have any effect at all when it comes to slowing down a descent. In fact, most times when I extend the spoilers, the engines spool up to maintain the same speed I guess. Even when the engines are at idle, the plane does not slow down. I'm almost never able to be at 250 knots or less when I descend below 10,000'.

Also, in a somewhat related question, I read that the preferred & most economical way to descend from cruise altitude is to have a descent with idle power. I have yet to see that in any of the flights I made. I'm not sure if PM CDU is not calculating properly or whether something is not quite the way it should be.

In a nutshell, the descents are always the most problematic for me. Is anyone out there able to control their descents properly (especially the speed), and if yes, could you please share how you do it?

Thanks,
Maurice

[BillTee]

Hi Maurice,

I too, use the PMDG models. The -800 is much more difficult to slow down than the -700.

I had a conversation with a Continental Capt where he said " it's not easy to go down and slow down."

Seems like the speed brakes don't seem to act on the -800 as much as the -700 PMDG ( must be inertia, as the -800 weighs much more than the -700 ), I just begin my descent farther out with a lower rate of descent.

Maybe you might have to play around with the aircraft config file to make it work more to your liking.

Let us know how it goes.

Bill T.

[mauriceb]

Thanks Bill. I'm going to try the 700 to see if that works better for me. There is still the issue of seldom seeing idle power only during the descent, especially when the speed setting on the CDU is much less than the actual speed. This may be a flight model problem, but I would think the CDU should certainly be commanding idle power descent once the speed is exceeded (or is that not a function of the CDU?)

Thanks,
Maurice

[stefanloss]

Actually there is a rule of thump that you can either slow down or descent, but not both at the same time.

[Westozy]

Hi Guys, I fly the PMDG 800NG and find the speedbrake to be quite effective especially in manual flight. Are you having an autothrottle problem? Is there a conflict between PM & PMDG? Just curious!

A tip I picked up from a visiting NG captain about descents - press the MTRS button on the EFIS to display the altitude in meters above the imperial altitude readout, use the first two digits as a guide in miles from destination to commence descent at flight idle.

[Bob Reed]

Westozy.. That is GREAT info! Another thing to check in the ini file of the MCP or CDU I do not remeber, make sure that the pilot controls the speed is turned on and not the AP.... If the throttles are at idle they will not spool up.

[mauriceb]

Actually there is a rule of thump that you can either slow down or descent, but not both at the same time.

I'm not sure that is true. I have felt a deceleration in a real plane when the spoilers are deployed during a descent, so they are effective to a certain degree.

Maurice

[mauriceb]

Hi Guys, I fly the PMDG 800NG and find the speedbrake to be quite effective especially in manual flight. Are you having an autothrottle problem? Is there a conflict between PM & PMDG? Just curious!

A tip I picked up from a visiting NG captain about descents - press the MTRS button on the EFIS to display the altitude in meters above the imperial altitude readout, use the first two digits as a guide in miles from destination to commence descent at flight idle.

This is a great tip indeed. But that means you don't let the PM CDU decide when the descent should start. If it is not calculating that properly, then the code is not doing what it should be doing. Also, if you want idle power, then you have to disconnect VNAV otherwise you can't control the power (since VNAV seldom commands idle power, for me anyway). In that case, VNAV would only be good for climb & cruise.

I do believe though this is at least partly an auto-throttle problem. As I said, I seldom see the power at idle for any descent that is triggered by the CDU. However, I have also noticed very little difference in the speed if the A/T is not on and I extend the spoilers after I retard the throttles manually to idle.

The power not being at idle during a descent I believe is the worst problem and I think FS9 is to blame for that, or the PDMG flight model if it models the same behaviour as FS9. There is no way there should be anything but idle commanded during a descent if the speed is already above the speed that is required.

If I was flying semi manually (with MCP only). I'm sure I could manage to not exceed the maximum speeds by controlling the start & rate of descent. And that is fine is that's what you want to do. But what is the point of having a CDU if you can never use its full ability.

By the way, I fly the PDMG-800 with no panels, so I can't see how there could be a conflict, but who the heck knows?

Maurice

[NicD]

Interesting! We've been experimenting with this during sim development. We too find that the PM CDU does not initiate descents that allow it to have the aircraft over fixes (or at 10,000) at or near the CDU commanded speed. Usually it is around 40-60 kts IAS faster.

From my experiments I believe that its caused by 2 things. 1) The PMDG 737-800 spoiler does have some effect but not as much as in the real aircraft, and 2) the PM CDU logic is oriented toward altitude control first, speed control second (which is probably same as real life it's just that the balance is not quite right).

When we're about to commence a tight STAR (ie no manouvering room to bleed speed on the profile) we use the commanded TOD as a guide but we manually commence the descent around 10 miles before TOD. Sometimes we also disengage A/T and pull the throttles back to idle for much of the descent, re-engaging A/T before the IAF on approach.

A 737NG skipper once told me that this bird is a slippery beast and speed control is one of the greatest challenges to master when they undergo type training/conversion (especially when coming from larger turboprops where the prop control range can be used to quickly create massive drag and reduce speed).

Agree with your thoughts on A/T idle Maurice ... doesn't make sense that it should be around 35% N1 when going too fast. Might be an engine preservation issue in the logic ... e.g. not safe or good for the engines to be at full idle for long periods airborne? Dunno - but I'll ask around and try to find out.

[mauriceb]

Sometimes we also disengage A/T and pull the throttles back to idle for much of the descent, re-engaging A/T before the IAF on approach.

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Agree with your thoughts on A/T idle Maurice ... doesn't make sense that it should be around 35% N1 when going too fast. Might be an engine preservation issue in the logic ... e.g. not safe or good for the engines to be at full idle for long periods airborne? Dunno - but I'll ask around and try to find out.

Nic,

I think you are right & disengaging the A/T for the descent and starting the descent early may be the only way to operate the SIM if the software does not do it properly. I guess that will keep me busier and in a way, it's more 'interesting' that way . I'm not sure if setting the MCP altitude at a certain value would stop the descent at that altitude as it does if VNAV is engaged or if the descent would continue until you select 'alt hold'. Will have to check on that one.

As far as 35% N1 minimum, you may also be right. Maybe too much cooling could damage the engines. If you find out for sure, please let me know. I'm very curious about that.

Thanks,
Maurice