737-800 Autothrottle Question

[parallel60]

Hello all,

The other day I was taxiing out and when cleared to position I went to engage the Autothrottle and it wouldn't arm.

-FD's were on.
-TOGA Takeoff
-CDU preflight was complete ie.ZFW and TO weight were entered.
-Unpressurized Takeoff after Deice.

I've asked some line pilots and only one person had that happen and suggested it could be temperature related, but wasn't sure. -15c at the time. Could it have something to do with the Isolation Valve being closed?

Through about 3000AGL after reconfiguring the Bleed Panel, the Autothrottle armed and worked the remainder of the flight.

Thanks for your time.

[B737900er]

There are several reasons why the autothrottle may not engage.

FMC preflight not completed
invalid OAT
Inop RA1
MCP solenoid switch INOP
EEC data bus fail.

[framer]

Air/ground logic can be a cause if the switches get dirty or iced up.

[parallel60]

B737-900er and Framer,

Thanks for the replies. It was snowing at the time and the previous flight picked up a lot of ice which required a prolonged de-ice. Airplane was fully serviceable with no MEL's at the time.

I had talked to Maintenance at destination and they had no suggestions specially with a serviceable airplane. Maybe the MCP solenoid switch is becoming intermittent.

Thanks.

[framer]

I have had a dirty weight on wheels switch cause a partial air/ground logic failure. The first thing that happened was the Autothrottle flicked off. Maybe in your case it was a bit sticky due ice?

[ImbracableCrunk]

Generators on line?

[parallel60]

Yes it could be ice related. Generators were online.

[B737-800W]

OAT in the FMC?
If you do not insert an OAT the A/T will not engage

[tony64peter]

Usually it occurs when airspeed is in excess of set value, 747 was 50kts. Taxi onto runway with highspeed entry and say 30kts of headwind you won't be able to engage the auto throttle until after T/O .

[parallel60]

B737-800W,

I'm thinking that you may be right. I may have forgot to enter the OAT on that leg.

Thanks for the reply's.

[172_driver]

Not intended as criticism at all,

But is it possible to obtain your target N1 for take-off thrust unless an OAT is entered?

[Flightaddict]

I think without OAT the preflight is not complete, I had a couple of times problem with A/T too and still unknown the cause, but I was able to engage it during climb.

[RAT 5]

//60. Will you pls explain the 'unpressurised takeoff' due de-ice? That's a 1st for me.

[rogerg]

Will you pls explain the 'unpressurised takeoff' due de-ice? That's a 1st for me
Under certain circumstances the engine cannot supply cabin bleed and anti icing. In this case the take off will be unpressurised to achieve the required engine perfomance.

[Pub User]

It's to prevent unpleasant fumes from de-icing fluid entering the cabin via the APU.

Quote from the 737 AFM:

Note: If Bleeds OFF take-off performance is required after de-icing, perform the ‘Unpressurized’ Take-off procedure, and ensure APU BLEED air switch is OFF.

[BOAC]

It's to prevent unpleasant fumes from de-icing fluid entering the cabin via the APU. - and engines. The same reasons some airlines go 'unpress' if the runway has been de-iced.

[tony64peter]

Further to my last the speed is 60kts. If your practice is to stand the thrust levers up, stabilise engines,release brakes, then push TOGA switches, often in strong headwinds, more than 60kts IAS will be indicated.You will not get N1 annunciation as there not enough time for the A/T system to set the N1 before 80kts. ie THRUSTHOLD. So manual setting is required.More common on bigger aircraft.

[Pub User]

- and engines.

I'm not sure if it's all that common to take off without engines, even if the runway is covered in fluid ;)

[BOAC]

Think loco?

[parallel60]

172 Driver,

CDU Preflight was completed for sure and we had a target N1 as I remember including this value in my breifing. However, I cant recall if the OAT was entered. But if the preflight cannot be completed without entering an OAT it must have been entered. Could possibly be one of those once in a while things or the MCP Autothrottle switch is about to go U/S.

As for unpressurized T/O after de-ice, our company's SOP is to perform this procedure(found in FCOM 1 Supplementary) to avoid fumes entering the cabin. Might not necessarily be a Boeing procedure.

Thanks all.