Decent with Project Magenta 737-800

[jr2mey]

Hello All,

Is there anyone out there that is using the Project Magenta aircraft they provide to use with their software? 

The issue that I am having is that everything works well EXCEPT decent.  When beginning my decent, with throttles at idle, even spoilers 100 percent, I am still only able to achieve about 1500 FPM decent.  Anymore, and my speed bug starts climbing again. I have tried manual decent and LVL CHG with my MCP.

Anyone ever experience this and if so, what did you have to do to fix it?

I do like this PM aircraft because of its extended range. Is there a better aircraft to use with PM software that is easily used and set up?

Is this an issue, or is it that I am performing my descents incorrectly and need to stop using my belly button as a porthole to look through...

Thanks,

[KyleH]

What airspeed  are you trying to descend at?

If the plane is left in the same configuration (thrust setting flaps, spoilers) you can't increase the descent rate without increasing the airspeed.

[jr2mey]

Hi Kyle,

When told to descend to a given altitude, I reduce my thrust to idle, and let my speed reduce before pitching the nose down. By doing this, I can only achieve 1500FPM decent rate, and to achieve even that I have my speed brake at maximum.

Congrats, by the way, on your check ride!

Thanks,

[KyleH]

Try not slowing down. Just bring the throttle back to idle and let it descend at the cruise airspeed.

[Bob Reed]

Are you using VNav?

[jr2mey]

Morning Bob, no I haven't been using VNAV.  I have been either doing it manually or using LVL CHG. Maybe I will see how that does.

That aside though, shouldn't the plane be able to descend better that 1500 FPM without having spoilers 100 percent and throttles at idle to keep the plane from continuously gaining speed?

Its also making it difficult to make many of the crossing restrictions given by ATC.   

[jackpilot]

In a descent (or climb)speed can be managed two ways
with Power and/or
with Pitch

AP and VNAV operations are well described in PMDG manuals (attached)

For manual descent from cruise, simply reduce power to idle or close to idle and let the airplane sink by itself, shoot for approx 1800 ft /mn to allow pressurization to keep up.
When stable, trim to hold that VS assumig speed is within parameters.

The fine art is combining pitch and power to stabilize your descent
You will find that small inputs work better.
Power reduction (small) will not really slow down the airplane but increase pitch which will translate into a  slight increase in speed.
Pitch reduction (slight nose up) will reduce speed  and  vertical speed
In other words remember that when you add a bit of power the aicraft will (slightly)acclerate but also pitch up (and slow down)
When you reduce power , the aircraft will (slightly) slow down but also pitch down (and speed up)
Remember that if you are too fast, pitching down will increae speed beyond limits (this is probably why you have to put your spoilers up 100% , which you should never do, as the spoilers flight detent is the max allowed).

Actually, at a constant power setting (close to idle in a descent) speed is controled exclusively by pitching up (slowing) or down (accelerating)

So, especially on approach, reduce speed first with power of course but by using a reduced VS or even  level flight. You will see a very rapid bleed of speed that way.

These changes are subtle and a good mix of small inputs to speed and pitch will place your aircraft on rails on its way down.
All this is subject to weight/weather/.. and flight model
Practice is the best teacher.

[Trevor Hale]

Hi James, Been a while since I was in my sim, however I do find the autopilot descends at a blanked v/s of 1500fpm.  I usually switch to V/S Descend and adjust my decent with the V/S Dial.  Just a thought.

Trev

[KyleH]

Morning Bob, no I haven't been using VNAV.  I have been either doing it manually or using LVL CHG. Maybe I will see how that does.

That aside though, shouldn't the plane be able to descend better that 1500 FPM without having spoilers 100 percent and throttles at idle to keep the plane from continuously gaining speed?

Its also making it difficult to make many of the crossing restrictions given by ATC.   

Again what speed are you trying to descend at?
This is an important question to answer as my feeling is you are not grasping the physics involved.

Assuming the aircraft configuration is not changed: Airspeed and descent rate are directly linked. You cannot get a steeper descent rate without also increasing the forward airspeed. Think of a car at the top of a hill with a normal grade; if you let off the throttle it will descend the hill at a certain speed lets say 30MPH. Now the same car on a steeper hill will accelerate more and faster to say 50MPH.

It's the same with aircraft.
(NOTE this is an example only, I don't have the actual numbers)
In your case the plane is configured with spoilers up, throttle idle, flaps and gear up.
It descends at 1500fpm at 200kts.
If you want to descend at 1600fpm, you need 210kts of airspeed.
If you want 2000fpm, you need 250kts
If you want 3500fpm, you need 300kts etc.

[jackpilot]

In your case the plane is configured with spoilers up, throttle idle, flaps and gear up.
It descends at 1500fpm at 200kts.
If you want to descend at 1600fpm, you need 210kts of airspeed.
If you want 2000fpm, you need 250kts
If you want 3500fpm, you need 300kts etc.

Could also be said differently:
Assuming you are at idle and clean config
(ie: you cannot play with power to slow down the aircraft , only pitch (VS) and config can)

To maintain 200 kts you have to adjust pitch for 1500fpm
If you want 2000fpm, your speed will increase to 250kts
If you want 3500fpm, your speed will increas to 300kts etc.
These figures can vary with flight models, weight, and weather (air density). as well as aitcraft configuration (use of slats, flaps, spoiler, gear)

One old trick to manage your descent:
Assume your are cruising at 33000'
your destination airport altitude is 1000.
You must then loose 32 thousands
32 X 3= 96
Start a standard descent 96 miles away from destination.
You can apply that from cruising altitude  to 10000 (where you have to reduce to 250kts before going lower) etc...
All in all, the key is planning or staying ahead of the aircraft, not reacting at the last minute to correct whatever is wrong.
ATC instructions are based on specific aircrafts performance, they are not supposed to ask for the impossible. So assuming your flight model is relatively accurate, you should'nt have problems complying, as long as you are planning your next move well before the aircraft knows about it ! lol 

[jr2mey]

Everyone, thanks for the quality of information.

Jack, your last post I think is the key to my problem. I wouldn't be starting my decent from 33k until about 60 miles from destination. I will use the rule of thumb you gave me next time, and then just request a decent if I don't get instructed to sooner.

I know that having to use spoilers like I did was a bad no no, but I was disparate to figure out why I couldn't comply with the ATC instructions and restrictions.

I can't try it this weekend but I will try next.

All of this thread has been a good read, thanks all!

[KyleH]

Interesting to note that the different ways Jack and I described the descent, also corresponds to the differences between the LVL CH mode and the V/S mode.
- LVL CH: you set an airspeed, and you get whatever vertical speed that results in
- V/S: you set a descent rate, and you get whatever airspeed that results in.

For more good information on descent check here:
https://jethead.wordpress.com/2013/10/18/common-sense-descents/

[jackpilot]

Absolutely right, and I think it is great to be able to grasp all that by flying manual, and then, only then, using automation to lighten the workload but with a good understanding of what's happening and why.
The big difference between a C-152 and a B-737 is inertia and how fast things are happening, but the above principles apply  to both.
Cheers
Jack

[Flying_Fox]

I like this insight from Jethead:

https://jethead.wordpress.com/2015/07/03/so-you-want-to-become-a-pilot/

"Even now as a captain with thirty years at the world's largest airline, I see copilots mystified by why some formula they use for descent or intercepts is not working out in realtime. I have only one answer: FTFA (Fly The F*ing Airplane). Because I don't know or care what component of the complex mix of time, speed, distance and altitude is screwing any formula, because again, we don't fly on paper–we fly in the living, breathing, ever-changing sky in a unique aircraft that resists the one-size-fits-all mentality of formulas and gouges."

[Joe Lavery]

I like this insight from Jethead:

ever-changing sky in a unique aircraft that resists the one-size-fits-all mentality of formulas and gouges."

Strange comment that, I only use gouges for wood turning, can't see a use for them in the cockpit.....

[Flying_Fox]

I like this insight from Jethead:

ever-changing sky in a unique aircraft that resists the one-size-fits-all mentality of formulas and gouges."

Strange comment that, I only use gouges for wood turning, can't see a use for them in the cockpit.....

apparently he meant  gauges, but who knows...