The art of FL CH and VERT SPD

[Richard McDonald Woods]

When flying the PMDG 747-400X I find that, during climb, I need to use these controls to force the aircraft to climb to its commanded cruise altitude. Otherwise it seems to become lazy, and just wallows at some lower altitude.  I believe this to be a shortcoming of the design.

Are these controls designed to be used in the real world aircraft only during cruise?

[Jeroen D]

No, they can and are used during different flight regimes. I have some notes at home from a KLM pilot on how he uses these.

I'm not sure I understand exactly what you are describing. The most common way of climbing would be on VNAV. When you say 'force', what mode are you using?

Jeroen

[Mundyas]

Hi

I might be off the point, it has happened before and I don't have PDMG.

I remember when I first tried to fly my plane was set up too heavy for the altitude. It took me a while to work this out!

One thing on I think VNAV cruise page on FMC. On RHS 4R shows the OPT & MAX Altitude Line. "Indicates optimum economic altitude and maximum altitude based on gross weight .."

Presumably your plane not to heavy for CRZ altitude?
Regards

[Richard McDonald Woods]

Yes, perhaps I should have been more helpful.

I was cleared to FL350 on my latest flight. At around FL300, although not at all overweight, the aircraft stayed at this altitude. To force it to climb to the assigned altitude, I set FL CH on, and set VERT SPD to 500. The aircraft then climbed OK.

I am not expecting an investigation into the inadequacies, or otherwise, of PMDG B744, but it caused me to wonder what real pilots use the switches for.

Any help much appreciated.

[Mundyas]

Interesting question Richard. It made me realise how little I know.

Just a few thoughts.

Not a real pilot!  I have been looking in my library of books.

Boeing 747-400 Checkride manual by Mike Ray - Helpful on landing. Non-Precision Approaches and Precision Approaches. (In my view an excellent book)

Big Boeing FMC Users Guide. Suggests Vertical Speed as "Very Special". ..."Does not have stall or overspeed protection"
This mode best limited to
1) Non Precision Approach.
2) Shallow step climb when carts in aisle ...or don't want to wake passengers.
3)Compliance with clearance "Descend so as to cross ABC and maintain FL230.

Emergency Descent. Set MCP altitude. Push FLCH Switch etc.
Regards

[Avi]

Hi,

FL CH and V/S are 2 different pitch modes. You can't have them at the same time.

I set FL CH on, and set VERT SPD to 500.

When you pressed FL CH you engaged FL CH pitch mode. When you pressed V/S you disengaged FL CH and engaged V/S.

FL CH tries to get to the target altitude in 2 minutes so if you make a 2,000 ft step climb, the commanded vertical speed will be relative low even at low altitude and low weight. This mode protects the aircraft from over speed or low speed.

In V/S mode the aircraft will set the pilot's commanded vertical speed and the aircraft can over speed or get low speed. It is the pilot responsibility to prevent problems or even disasters.

Cheers,

[martin1006]

@mcdonar
Can you be more specific about the conditions?

TOW....?
MCP alt set to ....?
Leg pages....? (are the any B constraints in the legs such as 300B?)

Can you reproduce the same situation?

How do you normally climb to your target altitudes when you get step climbs? (for example, if your target is FL360 but you get steps such as 5000ft, FL180 and FL240)

[ctdesing]

Hi, these two pitch modes are used a lot in real life, they are used more for descent. than for climb. That is because VNAV is restrictive to top of descent with idle thrust. If you select to descent with VNAV before the top of descent the airplane will start to descent at 1000 fpm to reach top of descent. In US very often the atc request you to descent as fast as you can, so VNAV 1000fpm to T/D may not suit you. There is when you need FL CH and VS.

Flight Level Change: pitch the airplane with idle thrust maintaining selected airspeed to reach the selected altitude.

VS/Vertical Speed: command descent or climb to maintain the selected vertical speed to reach the selected altitude.

[ctdesing]

This is from the 747 Flight Crew Operations Manual:

VNAV - Arm VNAV by pushing the VNAV switch (the light illuminates and VNAV annunciates on the PFD pitch mode anunciator in white characters below the current pitch mode). VNAV activates at 400 feet and provides pitch commands to maintain the FMC computed airspeed/path:
-VNAV SPD (active) - AFDS maintains the FMC speed displayed on the PFD and/or the CDU CLIMB or DESCENT pages. During speed intervention, use the MCP IAS/MACH selector to manually set the speed.
-When VNAV descent is initiated before top of descent (T/D) and the airplane subsequently intercepts the VNAV descent path, the pitch anunciator may change from VNAV SPD to VNAV PTH.
-VNAV PTH (active) - AFDS maintains FMC altitude or descent patch with pitch commands. For a non-entered headwind, thrust may increase to maintain the VNAV descent path. If the MCP altitude window remains set to current cruise altitude and the airplane is within two minutes of the top of descent, the CDU scratchpad message RESET MCP ALT displays.
-VNAV ALT (active) - If a conflict occurs between the VNAV profile and the MCP altitude, the airplane levels and the pitch flight mode annunciation becomes VNAV ALT. The airplane maintains altitude. Maybe that happened to you. To continue the climb or descent, change the MCP altitude and push the altitude selector or change the pitch mode. If below the VNAV path, resetting the MCP altitude window and intercepting the VNAV path will also continue the descent.

V/S - Pushing the V/S switch opens the vertical speed window and displays the current vertical speed. It also opens the IAS/MACH window (if blanked). Pitch commands maintain the rate of climb or descent selected in the V/S window.

FLCH SPD - Pushing the FLCH switch opens the IAS/MATCH window (if blanked). Pitch commands maintain IAS/MACH window airspeed or Mach.

[Richard McDonald Woods]

Many thanks for all your inputs. They have been a great help.

In summary:

1. VNAV activates at 400' and provides pitch commands to maintain the FMC computed airspeed/path during initial climb until TOC. During cruise, VNAV PTH maintains FMC altitude with pitch commands. During descent, VNAV will aim at idle thrust descent to minimise costs. Commands to descend before TOD is reached, using VNAV and MCP altitude settings, will cause a 1000 fpm descent until the aircraft intercepts the idle thrust descent path and to then follow it.

2. Intervention into VNAV control may be done using either FL CH or VERT SPD modes.

3. FL CH mode may be used for routine FL changes from level flight. It is safe to use because, although the aircraft will attempt to reach the selected FL within 2 minutes, the FMC will protect from low/over-speed conditions through pitch commands. This may result in achieving the commanded FL in longer than 2 minutes. FL CH for emergency descent should therefore be used to prevent over-speed conditions.

4. VERT SPD should only be used for shallow step climbs during cruise. It maintains the commanded descent/climb rate but without stall/over-speed protection. It can be used to ensure that the gin and tonics and sleeping passengers are not disturbed.