B777 V/S Mode

[Haroon]

hi

looking for some write up on the following mode of B777:

Condition 1: Maintaining an altitude e.g. 1000 feet with "ALT" indicated on FMA.

The aircraft does not descent in V/S below 1000 if altitude set in MCP altitude window is 1000 or above.

Condition 2: Maintaining an altitude e.g. 1000 feet (which is also the MDA and is set as minimums on the PFD).

The aircraft descends in V/S below 1000 even though the altitude set in the MCP altitude window is 1000 or above.

Is this logic (if I am correct) mentioned any where in the Boeing manuals?

thanks

[8che]

Its not mentioned in the manuals because its absolute garbage.

There is no logic associated with v/s mode whatever is set in the MCP.

[Wizofoz]

Condition 1)- The aircraft will not leave the MCP alt in V/S. It will revert to Alt. IF the MCP is ABOVE the actual altitude, the aircraft WILL descend in V/S.

Condition 2) Not sure what you're getting at- if the aircraft descends in V/S to the altitude set in the MCP, it will capture that altitute. There is no relationship between what is set as the MDA and the behaviour of the AFDS.

[Haroon]

Thanks Wizofoz

IF the MCP is ABOVE the actual altitude, the aircraft WILL descend in V/S

thats what i was looking for. do you know a reference about this feature in the boeing manuals?

thanks

[de facto]

Be nice now

[Wizofoz]

Not specifically, but it's simply that the feature works by attaining the commanded V/S until it captures the MCP alt.

If the MCP ALT is above you and you are descending, it never will, so it KEEPS descending.

It does also have over-speed and low-speed protection features.

[Haroon]

Thanks guys

Ref: 11.31.30

When the FMC is “on approach”, the following features are available:

• The MCP altitude can be set above the airplane altitude for the missed approach. When the MCP altitude setting is at least 300 feet above the current airplane altitude, VNAV continues to command a descent.

Just like the above reference I am interested in finding out where in the manuals it is written that "IF the MCP is ABOVE the actual altitude, the aircraft WILL descend in V/S".

I know it does! there is no confusion about understanding this (1000 feet was just an example to explain what I wanted to ask).

Since it is a feature, it must be mentioned somewhere like it is for the VNAV case.

Could not find it in the flight management navigation or approach procedures.

Regards

[Haroon]

okay thanks everyone

[Wizofoz]

....And as such is probably something of an omission on Boeings part, as I'm pretty sure there are other types (does the 737? It's been a while!!) that WILL fly away from the MCP Alt in V/S.

[de facto]

....And as such is probably something of an omission on Boeings part, as I'm pretty sure there are other types (does the 737? It's been a while!!) that WILL fly away from the MCP Alt in V/S.

Condition1:
The Aircraft is level at 5000 ft, MCP set at 5000ft.
Pilot selects VS and tries to get a descent,other pilot looks at him/her and starts laughing.

Condition2:
Aircraft is in descent in VS mode towards a lower MCP altitude,if the MCP is reset to an altitude setting higher than the aircraft actual altitude ,the VS will now be in an OPEN DeSCENt mode,therefore NO protection from the automatics.the other pilot will start yelling
Pilots proper procedures for MCP setting,calling FMa changes is the protection..(ie CDA,approaching MDA)
.
The VS needs a minimum of 100 ft mcp change to arm the mode.

[Haroon]

Condition 3:
The aircraft is level at MDA and does a circle to land with higher (missed approach) altitude set in MCP. When ready to leave the MDA the pilot uses V/S to start the final descend. Other pilot neither yells nor laughs but just wonders where is this written

[Mr Good Cat]

The point I was actually trying to make was that V/S is a dangerous mode to use

I think that's a little bit extreme.

It should be used sensibly and monitored carefully, as should ALL modes.

Let's remember that V/S is the recommended mode for a climb in severe turbulence so it pays to practice it's effective use from time to time (such as on a CDA etc).

[Three Wire]

V/S is more correctly a transition mode between a climb/descent and a level altitude. It will take the aircraft away from an MCP ALT and it does have overspeed and low speed protection.
It is not particularly good for a CDA nor a holding pattern descent, just useful.
Never heard of V/S mode being endorsed by Boeing for climbs/descent/cruise in turbulence. Sounds like somebody's bright idea.

#W

[misd-agin]

"It is not particularly good for a CDA nor a holding pattern descent, just useful."


Use V/S exclusively during CDA's. Every pilot I've flown with does also.

Also use it during minor altitude changes (holding patterns, 1000' altitude changes, etc)

Also use it during climb/descent when ATC says "expect higher/lower in X minutes/X miles". Rather than blasting at climb/idle power to level off and then resume the climb/descent many folks prefer the technique of reducing rate of climb/descent to avoid an unnecessary level off. "Technique only"

[de facto]

. When ready to leave the MDA the pilot uses V/S to start the final descend. Other pilot neither yells nor laughs but just wonders where is this written

Nowhere.

Never heard of V/S mode being endorsed by Boeing for climbs/descent/cruise in turbulence. Sounds like somebody's bright idea.

#W

Neither have I.It is obviously better when entering strong turbulence to reduce the pitch by VS to lets say 500 a min while selecting the appropriate speed than going along for the ride with possibly quite high rate of climb.

"It is not particularly good for a CDA nor a holding pattern descent, just useful."

Why not? Are we talking about a CDA at low levels right?and why not during holding descents?

. Rather than blasting at climb/idle power to level off and then resume the climb/descent many folks prefer the technique of reducing rate of climb/descent to avoid an unnecessary level off. "Technique only"

Technique yes but boeing recommendation too,,less than 2000 ft change of level,VS is the mode...more than that,vnav or LVL change.

[Wizofoz]

ER- yeah.

UNLESS you are on an LNAV track and can use VNAV PTH, what OTHER mode would you use to achieve a CDA?

[Mr Good Cat]

Never heard of V/S mode being endorsed by Boeing for climbs/descent/cruise in turbulence. Sounds like somebody's bright idea.

Neither have I.It is obviously better when entering strong turbulence to reduce the pitch by VS to lets say 500 a min while selecting the appropriate speed than going along for the ride with possibly quite high rate of climb.

Fair enough. But my 777 FCOM supplementary procedure 16.16 (Severe Turbulence) says as follows:

"Climb, Cruise, and Descent Considerations
After takeoff, and when established in a clean climb configuration, use of
the autoflight system is recommended for flight through turbulence.
During climb and descent, use of VNAV or flight level change may result in
excessive pitch changes as the AFDS attempts to fly speed with the
elevators. Therefore, vertical speed mode (speed on autothrottles) is
recommended for climb and descent in severe turbulence.
During cruise, VNAV and altitude hold modes both fly speed on
autothrottles and can be used in turbulence."

[Mr Good Cat]

ER- yeah.

UNLESS you are on an LNAV track and can use VNAV PTH, what OTHER mode would you use to achieve a CDA?

Well, technically you could use FLCH and wait for the A/T HOLD mode then position the levers manually as per FCTM... but this gets funny looks from the F/O as you tweak back and forth to maintain the desired path/R.O.D...

[Poire]

Good Cat, do you have the FCTM reference for that technique?
(A/T in hold, set thrust manually ... )
Appreciate it

[Mr Good Cat]

Good Cat, do you have the FCTM reference for that technique?
(A/T in hold, set thrust manually ... )
Appreciate it

My apologies - I meant to say it's in the FCOM (not FCTM):

FCOM 4.20.5

Autothrottle Modes
The autothrottle modes are:
• THR – The autothrottle applies thrust to maintain the vertical speed
required by the pitch mode.
• THR REF – Thrust is set to the selected thrust limit displayed on EICAS.
• IDLE – Displayed while the autothrottle moves the thrust levers to idle;
IDLE mode is followed by HOLD mode.
• HOLD – The thrust lever autothrottle servos are inhibited. The pilot can
set the thrust levers manually.
• SPD – The autothrottle maintains the selected speed displayed on the
PFD. Speed can be set by the MCP IAS/MACH selector or by the FMC,
as shown on the CDU CLIMB, CRUISE, or DESCENT page. The
autothrottle will not exceed the operating speed limits or the thrust limits
displayed on the EICAS. If only one thrust lever is engaged, "L-" or "R-"
displays in front of SPD for the engaged thrust lever.