The manual states "A VNAV cruise
descent is commanded at the current cruise speed and approximately 1250
feet per minute rate of descent." 
I have followed the procedures as set out in the manual and when I execute the throttles retard to idle and the descent rate is 3000fpm not the 1250fpm as above. Is anyone else having the same experience?

Many Thanks
Chris

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

Is anyone else having the same experience?

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

Liam

Thank you for your response. I agree if you use the descend now function then the behaviour is as you describe and as expected.

However the manual states that for descents starting more the 50Nm before the Top of Descent point should be initiated using the cruise descent. I looked this up in the manual and its says

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

When I follow these instructions that is when the aircraft pitches down to 3000fpm until it captures the new cruise altitude. Its not what is described and I thought I maybe setting things up incorrectly.

Thanks

Chris

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

Chris,

Apologies, I misinterpreted the original post (and the title, it seems ::), not my day!) as concerning a usual descent procedure from cruise altitude.

I have just tested the CRZ DES function from FL360 to FL340, 94nm from T/D.
The screenshot shows -3000fpm, so this does violate the manual at page 443.

I believe another more minor issue here is the automatic scratchpad input of the new MCP altitude.
The scratchpad input is appearing only when increasing the MCP altitude above current cruise altitude, but not below such as in this case.
---
(757 III part 2, pg 344):
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.
---

I will file a bug report for the excessive descent rate.

Can you also reproduce this scratchpad issue?
When lowering the MCP altitude to the new lower cruise altitude, the scratchpad does not show it.

Thanks.

---

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

The scratchpad issue, the way you describe it, has always been present in the Captain Sim 757 v4.x/757 III, and 767 v1.x/767 II. And my question is if it should happen at all because:

In the other developers Boeing 747 QOTS II, and 777-200ERF, this doesn't happen with either an increase, or a decrease, in the altiutude selection.

:-/ :-/ :-/

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

Funkarter wrote on May 25^th, 2020 at 8:16am:

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.

I agree, since the CS 757 manual states the following for the DES NOW function:
"Once the descent is established the autothrottle mode changes to THR HLD to allow the pilot to adjust the rate of descent with power changes."

The cruise descent version reads "Thrust levers can be manually positioned to adjust the descent rate", suggesting that indeed THR HLD would be the correct FMA for this flight phase.


I also found a 777 FCOM online which described FMA behaviour:
---
"An early descent is any descent initiated prior to the T/D point. There are two
types of descents that occur prior to T/D: a “Cruise Descent,” and a “Descend
Now.”
Both descents are identical in terms of initial descent parameters; they differ
only in their effect on the existing descent path. The initial parameters are:
· Rate of descent: approx 1,250 fpm
· Autothrottles: initially THR until approx 1,250 fpm, then HOLD
(allowing the pilot to adjust the rate of descent as necessary)
---

If the procedure is similar for the 757, then perhaps the thrust mode is EPR until ~1250fpm descent is established, then THR HOLD beyond that.

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

my question is if it should happen at all because: In the other developers Boeing 747 QOTS II, and 777-200ERF, this doesn't happen with either an increase, or a decrease, in the altiutude selection.

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.