MD-11, PROF, and ATS system question

[DAL2728]

I was talking to an MD-11 pilot the other day who flies for World Airways and he told me it was company policy to arm NAV on the ground but PROF at 400ft AGL. When I asked why they didn't arm PROF on the ground as well he told me that if PROF is armed the throttles are advanced too far forward the ATS might interpret it as a takeoff.

I didn't think to ask for more details of him at the time but perhaps you all might help. My question is, at approximately what power setting will the ATS clamp the throttles for takeoff power? It was my understanding that breakaway thrust for the MD-11 is about 40% N1 with any of available power plants. If you're simply taxiing the aircraft is there that much of a possibility of the power running away with you and setting to takeoff if PROF is armed on the ground?

Thanks!

[DAL2728]

Bump anyone?

[rhovsquared]

what is 'PROF'?

ATS=autothrottle system ?

and [unrelated] do all MD-11's have PCA or just some wonderful idea, I miss McDonell Douglass, and Douglass Proper immensely :(

[bigbeerbelly]

The ATS will not engage on the ground until Autoflight is pressed. At and above 400ft pressing PROF will engage the ATS if it is off. My understanding is that PROF can not be engaged on the ground because it is inhibited. Most likely due to the fact that you do not want to be flying an FMS climb profile during during First segment performance incase of an (or dual) engine failure. So you climb at V2+10 until 400 then engage PROF.

BBB

[DAL2728]

Doesn't pressing Autoflight enable the autothrottle system? Something has to be there to set the takeoff power limit, right?


You're saying you can't arm PROF on the ground? I was watching the ITVV MD-11 video and they armed both PROF and NAV while taxiing.

[Capt Claret]

I believe the FMS on the B717 to be ex the MD11, and therefore assume that PROF & ATS have the same heritage.

The 717 FCOM indicates take-off should be in PROF and if cleared via a SID from the RWY, NAV.

The only time I've taken-off out of PROF have been those occasions when I forgot to arm it! :\

[DAL2728]

Captain, thanks for your response! It's frightening how much the flight deck systems of the 717 mirror the MD-11. This might be helpful.


So let's run with what you're saying. Prior to takeoff, you arm NAV, PROF, right? At what point does the PNF press the autoflight button? It's my understanding that when you press autoflight that also engages the ATS.

Once you advance the throttles to your initial power setting, does the crew press autoflight and the ATS takes the throttles and sets to takeoff clamp?

[Capt Claret]

G'day DAL2728,

PROF (& NAV if appropriate) armed during taxi.

The PNF presses AUTOFLIGHT when the PF is satisfied that both engines are accelerating uniformly - usually <30kias, this engages the autothrottle and sets take-off power, either flex or ref as set in the FMS.

Once airborne and at or above 400' AAL, the next press of AUTOFLIGHT will engage the autopilot.

rhovsquared

In simple terms PROF is the vertical flight path management of the aircraft. For climb it is usually pitch for airspeed coupled with climb power. For descent, it is usually pitch for a 3 degree descent to the runway from cruise altitude, with slow down segments to allow for speed reduction from descent speed to 250KIAS < 10,000' and then reconfiguring for landing.

[rhovsquared]

Thanks Capt Claret,

rhov :)