Radio/baro altimeter use on TO 737 NG

[Driver 170]

I know the radio altimeter is used for landing, but is it also used during the initial part of the takeoff? Or do you just use the baro altimiter?

[Gulf Julliet Papa]

RA is not used at all for Take Off, there is no point. All changes occur at height above the airfield so subject to operator they will use QFE or QNH and adjust for airfield height. For example if you are going to bug up at 1000ft above airfield and the airfield is 200ft AMSL.... then in QFE use 1000ft, if QNH 1200ft

RA (again subject to operator) is only used on landing for....
- Autolands
- GPWS callouts (2500, 50..40..30.. etc)

In the operator I work for we use QNH for all approaches and all configuration changes etc.

[Driver 170]

I'm not an NG pilot but i use the PMDG 737 NGX and the FCTM page 3.3, takeoff profile - https://www.dropbox.com/s/tc8hylzb7fegjam/Photo%2020-01-2015%2022%2051%2011.png?dl=0

It mentione at 400 feet RA engage a roll mode!

Thats where my confusion came from, as the FCOM and FCTM keeps mentioning RA and AGL.

So even in the FMS takeoff page 2 you have the CLB THRUST and Acc Altitude boxes and those inputs are AGL? Sorry about the words i use, i'm not a proffessional lol

So the RA is only use for landing GPWS and those callouts 2500 etc

[vfenext]

RA is not used at all for Take Off, there is no point
The RA is very useful on takeoff as a reference for when to start actions associated with a failure. Usually this happens above 400/500 AGL, therefore reference to the RA is easier than doing mental maths with the baro. It's especially useful at high altitude airports. In essence if it's there use it!

[Driver 170]

So during the takeoff profile you use the Baro Altimiter! Now the FMC uses AGL for takeoff page 2 entrys for the Climb Thrust etc. Is that only for the FMC use and the PF should use the Baro ?

Cheers

[Denti]

Those entries on T/O page 2 are above AGL, but the FMC will use baro altitude for those, referenced to the database altitude of the take off runway. Remember, for NADP 1 you use 3000' AGL for acceleration anyway which is above the max displayed height.

[Driver 170]

Hi denti thanks for the reply!

Em, what do you mean the FMC will use baro altitude for those? :confused:

Yeh for NADP1 your A/C is going to be higher than the RA 2500. So the alt is read off the baro and it'll ACCEL and nose down at 3000 AGL but thats taking off the Baro altimeter, is that correct? Or have i missed your point?


On my dept the entrys entered in takeoff page 2

ACCEL HT 3000 AGL
REDUCTION 1500 AGL

For NADP1

All happens at RA not baro alt?

[Driver 170]

For the last part i should say, 1500 REDUCTION happens at 1500 RA! As for 3000 thats on the baro alt.

[glendalegoon]

excuse me

on some 737s doesn't the RAdio Altimeter trigger a reduction in hydraulic pressure to the rudder actuator to avoid a full fledged rudder hardover?

I thought it was at 800'RA/AGL.

this is an automatic, non pilot induced function.