V/S oscillations in VNAV SPD mode

[Balt]

Hi all,

Situation: FL200, TAS 380, AP in LNAV, ALT, A/T in THR REF mode (happy to provide situ file). LNAV has next altitude target set to FL280, speed in FMS is set to 205 (yellow arc is 265, so that's the lowest target being set).

From this stable, level flight, activate VNAV mode, and simultaneously move the speed target to M.800. AP goes into VNAV SPD mode, THR REF remains and Max CLB PWR is applied. Then the FD is chasing in pitch, and major V/S oscillations result. I counted 5 oscillations with increasing magnitude to +1250/-1900 fpm then another 5 with decreasing magnitude until the target speed has been reached and the oscillations diminish in a climb.

Cheers

- Balt

[Hardy Heinlin]

Hi Balt.

(happy to provide situ file)

Yes, please.

Speed 205? Yellow arc?


Cheers,

|-|ardy

[Balt]

Speed 205? Yellow arc?

FMS speed is set to 205. Happy to call it the yellow ribbon if that suits... been flying arc instrumented airplanes only for almost 20 years...

This btw is from your Nakhchivan to Baku sample situ.

Cheers

- Balt

[Hardy Heinlin]

This btw is from your Nakhchivan to Baku sample situ.

This situ is designed to demo a special hold that will auto-exit when the aircraft passes FL150. In order to arrive at the hold below FL150, the OAT is set to 40°C, so that the climb performance is very low. And the QNH is set to 1041 hPa so that the geometric altitude of FL150 is higher, and it takes more leg distance to reach that altitude.

The speed constraint in the legs is 205 indeed, until after the hold.

CRZ ALT is FL280.

I have never tried to continue this scenario after the hold.

When the climb performance is very poor, ranging from, say, 0 to 500 fpm, and the airspeed is controlled by the elevators, the elevators will not have much control. As you know, at 0 fpm, the elevators have no airspeed control, from 1 fpm on, the control efficiency rises exponentially, and the trend to oscillations is initially very high until reaching a reasonable climb rate.


Cheers,

|-|ardy



The speed constraint of the hold causes the FMC to apply SEL SPD mode (205). To get out of this mode you need to select ECON manually (or whatever mode you want).