FLCH Takeoffs
 

We seem to always takeoff with VNAV armed but we did train in the sim for normal takeoffs without VNAV and then at an appropriate altitude, a selection of FLCH and selection of a target climb speed. Then a press of a button to reduce thrust to climb.

Has anybody had need to do this on the line.

Not once in 8 years/4600 hours on type.

Hi JammedStab,
For the first 26 years we flew aircraft which were not permitted to use flight directors for take off and didn't have a VNAV function anyway.
B737-200 was operated the way you describe.

We never used the FD for takeoff in the 737-200 either, but in the case for this thread, the FD is used in TOGA mode but without VNAV armed.

But what reason could there be for doing this on the line unless you were to have some concern about the validity of VNAV. I believe that this results in not having FD guidance for engine failure scenarios encountered below the acceleration height.

We never used VNAV on T.O. Boeing say you should consider not using VNAV if the weathers a bit dodgy, relating to windshear. There are also some software issues in the FMC which somehow relates to VNAV and windshear. This isn't the full story, but it's something along these lines.

Also a useful procedure if your contingency procedure states an early turn with V2 and you want to accelerate and clean up after turn is completed :ok:

Probably to keep you mentally awake during the different flight segments below the final climb segment.

WAY too many buttons to push! Simply open up the speed window!

In our 774/748 fleet we ALWAYS use TOGA with VNAV armed for takeoff. I can remember only 2 instances in 10 years on the 744 when TOGA failed to engage and I had to place the thrust levers manually.

That'd be the reason why, when you need to, it's stuffed up.

Now, now Bloggs.

The question was not has any one done this on the line! :rolleyes:

Since VNAV is prohibited (B744) when using QFE, I have used it on take-off from Moscow several times, and in Bishkek years ago when they still used QFE.

Exactly. We always takeoff with VNAV armed except when taking off QFE (or VNAV deferred).

Depending on the aircraft, armed vs engaged happen at different points/altitudes.
A coded DEP will always assume armed.

Never done it before. I once had to handle the levers manually as TO mode did not start. Or maybe inop all before the flight, still didn't read anything in the documentation. There was no need to cancel the take off when you still have hands to push throttles and keep N1 stable. But never FLCH, never heard or used before.

Anybody know why LNAV and VNAV is prohibited on the 744 when using QFE?

Answer discovered later...

Limitation on LNAV/VNAV with QFE is that any conditional altitudes in the FMS are referenced to QNH, not QFE. Secondary (Minor) consideration is airplane without GPS and doing DME updates will get slant angle/range wrong using QFE. In reality, if the departure doesn't have conditional waypoints, and the plane has GPS, VNAV will work without any issues for the very short time you are below the transition altitude.

FMC does not know altimeter is on QFE, so a conditional alt is with a condition, like after reaching 4000' turn left, where you can't manually code the point in yourself. Something like 5000B at a specific point you could actually could make work, as you could adjust the altitude and change it 4000B, (assuming 1000' field elevation using QFE)


For LNAV, the NAV accuracy for non-GPS planes is degraded by the slant range error.

In reality, most planes have GPS, or the slant range is not going to make a big enough error to really make a difference in LNAV unless you really need a low RNP.

Intruder.......who is pushing any buttons here? If in VNAV that would be the case though in this scenario.

Rojer Clarrie, my mistake. It is good fun though, taking off in white... or so I've heard! :}

I've heard similar Bloggs, I've heard similar. :8

If you actually tried to take off in FLCH (to do the low-speed turn as you describe), you'd have to manually position the throttles for takeoff and set the initial heading (which would have been done automatically in TOGA). Then you would have to engage FLCH and LNAV (or manual headings) after airborne and engage the autothrottles. Finally, you'd have to adjust the speed in the window from V2 to V2+10 (or your normal climb speed).

By contrast, if you armed LNAV and VNAV and used TOGA for takeoff, all you have to do is open the speed window after the speed bug jumps from V2 to initial climb speed. Autothrust and Flight Director takes care of the rest.

In the venerable Douglas/Boeing 717 ✈️ as flown by Bloggs, the system is a little different if using Level Change.

At or above 500' AAL, the PM selects AUTOFLIGHT & LEVEL CHANGE (pulls the level knob). This engages the autopilot and changes auto throttle from TAKEOFF. CLAMP, to TAKEOFF THRUST, which then allows the auto throttle to select climb power at the appropriate point. The aircraft is climbing at V2.

At ACCEL, the PF pulls the speed knob, initiating acceleration from V2 to enroute climb.