Funkarter wrote on May 23^rd, 2020 at 7:48am:

Hi Chris,

Are you using the DES NOW function in the FMC, or descending at the usual T/D point?

In the initial stage of descents from T/D, I also experience quite high descent rates.

An early descent with DES NOW works correctly at -1250fpm for me, but if starting at T/D, the aircraft will wait for the diamond on the deviation scale to show the aircraft is slightly too high on the VNAV path before it pitches to begin descending.
I then experience a higher descent rate (between -3000 and -4000fpm) as the aircraft catches up with the path on the scale.
Once back on vertical profile, the descent rate usually becomes more sensible.

Is this what you experience?

Cheers

Ah, I didn't consider the CRZ DES option.

I haven't used that method for a long time, however I do vaguely recall steep descents as you describe.
I'll test this again and report back.

Cheers

Liam

I can confirm that I have the same scratchpad issue. It pre-fills going up but not when entering a lower altitude.

Another observation is that the 767 shows the same behaviour when using cruise descent. Interestingly the 767 manual makes no reference to a V/S of 1250fpm. It may be an error in the 757 manual.

Chris

Hi.

I have searched the internet and found a Boeing flight crew training manual from 2010

Extracts from the document are:-

CRUISE DESCENT
Resetting the MCP to an altitude below the current cruise altitude causes the new altitude to be copied to the scratchpad if the altitude change is 4000 feet or less. The new cruise altitude can be entered on the cruise page. When the modification is executed the CRZ page displays ACT ECON CRZ DES. If the altitude set in the altitude window is below the speed transition (SPD TRANS) or restriction (SPD RESTR) altitude displayed on the DES page, those altitudes and
speeds are deleted and the airplane will maintain cruise speed during the descent.
Note: A cruise descent will not provide speed adjustments to comply with speed restrictions or transition altitude speeds. Transition or speed restrictions must be maintained by flight crew action.

This text is also found on page 344 of CS part 2 manual.

This appears to confirm the scratchpad issue valid.

Next extract (Also page 443 of CS part 2 manual)

CRUISE DESCENT
Cruise descents can be started in the cruise phase when the airplane is more than 50 miles from the T/D point.
A cruise descent can be started by selecting a lower altitude on the MCP, entering the new altitude in the CRZ ALT line and executing. A VNAV cruise descent is commanded at the current cruise speed and approximately 1250 feet per minute rate of descent.

The auto throttles adjust thrust to maintain the target descent rate; pitch maintains the commanded speed. Thrust levers can be manually positioned to adjust the descent rate.

A further Boaeing document from 2008 states that a typical rate of descent for 757-200 in a clean configuration and throttle idle is 1800fpm. With speedbrakes deployed this increases to 2700fpm.

Therefore my only observation is that the evidence points towards the behaviour being experienced of a decent rate of 3000fpm in a clean config is too high and incorrect.

Besides the vertical speed being incorrect the throttle  mode is also appears to be incorrect as it indicates Idle not THR HLD, which should allow the flight crew to manually adjust the throttle.

Chris

Markoz wrote on May 25^th, 2020 at 4:01am:


Hi Mark,

Having just flown the other developer's aircraft, I agree that the scratchpad altitude does not display in the 747 QOTSII. In the provided FCOM, cruise climb/descent is said to be performed by the altitude intervene switch (and also in the 777ERF).  This fills the altitude field automatically on the VNAV cruise page.

Since the 757/767 does not have a push button on the ALT dial, I assume the alternate method Boeing used for "automatic" input was to copy the altitude into the scratchpad.

I will file a bug since the manual states it as a feature, and it is only partially implemented as it stands.

Thanks.