E/O DD path must not be shallower than 300 fpm

[Hardy Heinlin]

Good morning,

I missed a statement in the official manuals:
The system assures that the E/O drift down rate will not go below 300 fpm.
It may be 500 or 900 or more fpm. But not less than 300 fpm.

As we all know, when max CON thrust remains set, and LRC or E/O speed is maintained, the descent rates for the next two hours will go like this (for example):

900 ... 600 ... 400 ... 300 ... 250 ... 230 ... 220 ... etc. ... 120 ... 115 ... etc.  ... 79 ... 78 ... 77 ...

It's an exponential curve. Not linear.
In the end, with the thrust still at CON, half an hour will be spent with rates less than 300 fpm.

ATC will not allow rates less than 300 fpm.

So when the rate decreases below 300 fpm, VNAV SPD will push the nose a little further down.

We know what will happen: The airspeed will increase.

When this happens, the system has two options:

(A) Allow the airspeed to go above the speed bug and accept any overspeed
(B) Reduce the thrust slightly below CON limit

I say (B) is the only option. The thrust will stay at CON for a long time at high altitudes. And when the descent curve gets into the 300 fpm region at lower altitudes, the thrust will be slightly reduced below the CON line -- it's just a few millimeters; hardly noticeable.

The 300 fpm path will positively end at the VNAV PTH capture altitude, and the thrust will re-increase to maintain level flight.

The fact that the level flight requires a higher thrust than the descent flight (at the same density altitude) proves:
The thrust in the final 300 fpm descent region needs to be lower than the thrust at level flight at MAX E/O ALT.

Aside from that, the aircraft must be pitch stable at level flight, i.e. there must be some thrust reserves for airspeed corrections, and some elevator authority for altitude corrections. This is the second proof that the thrust needs to be slightly reduced in the final segment of the 300 fpm descent.

Am I missing something?

The third option would be: Ignore ATC's 300 fpm limit and descend at rates around 2 fpm for three hours until the target altitude is reached -- if it ever reaches it at all (reality and FMC theory do not always agree).


Regards,

|-|ardy

[Hardy Heinlin]

One more point:

When descending for two hours at rates less than 300 fpm, the gross weight in the end will be lower than it was when the E/O MAX ALT from two hours ago has been transferred from 5R to 1L. The CRZ ALT in 1L cannot be reached with a max thrust descent because the decreasing gross weight increases the E/O MAX ALT during these hours, causing a level-off at even higher altitudes (above the CRZ ALT entered two hours ago).

The only way to reach the CRZ ALT is to keep the descent rate at 300 fpm or greater. And that needs a slight thrust reduction in the end.

[simonijs]

Good morning,

KLM's AOM, VOL I, states:

"VNAV provides driftdown guidance if the aircraft is more than 150 ft above the maximum altitude. It results in a shallow descent, particularly when the aircraft approaches its maximum altitude.
If the descent rate becomes too shallow and the airspeed exceeds the target as a result of excess thrust, the pitch will be changed for a 300 fpm descent rate while auto throttle maintains speed. Mode annunciation will transition to SPD and VNAV PTH."
The normal FMA indications during the D/D would be THR and VNAV SPD.

A diagram is included in the text, dealing with driftdown. For a driftdown from an altitude above the Engine Out Max Alt, the aircraft should finally get to "ACT E/O LRC CRZ" or "ACT E/O CRZ" (if E/O SPD was executed). I noticed that I never seemed to get there, indeed taking hours to get lower and ending up too high at the FMC calculated T/D for a diversion. I was about to write a post on this.

I also looked at ICAO DOC 4444 (Procedures for Air Navigation Services - Air Traffic Management), § 15.1 for a description of Emergency Procedures. In 15.1.1.1 it says: "The various circumstances surrounding each emergency situation preclude the establishment of exact detailed procedures to be followed (...)". And in § 15.1.2: "An aircraft known or believed to be in a state of emergency, including being subjected to unlawful interference, shall be given priority over other aircraft".
An Emergency Descent is described in detail (even phraseology) but I guess here they mean descents well in excess of 300 fpm.

Kind regards,
Simon

[Hardy Heinlin]

Hi Simon.

KLM's AOM, VOL I, states:

"... If the descent rate becomes too shallow and the airspeed exceeds the target as a result of excess thrust, the pitch will be changed for a 300 fpm descent rate while auto throttle maintains speed. Mode annunciation will transition to SPD and VNAV PTH."

Thank you! That's the detail I was looking for. 300 fpm will be controlled by SPD | VNAV PTH, not by THR | VNAV SPD. Other manuals just mention the 300 fpm limit without further details on AFDS modes.


Regards,

|-|ardy

[Hardy Heinlin]

The 300 fpm descent rate limit logic is now implemented in PSX update 10.36:

http://aerowinx.com/board/index.php?topic=4191.0


|-|ardy