B737 loss of system B and Standby and PTU operation

[Banana Joe]

In the scenario of loss of System B Hydraulic System due to engine driven pump failure and electric motor-driven pump and Standby hydraulic system unavailable, would the PTU still be able to extend the LEDs despite the alternate flaps system operation since there is still fluid in the system B reservoir and pumps?

[FlyingStone]

PTU only operates the autoslats and the LE flaps/slats. In case of alternate extension, the LE devices are operated by standby system. And at the first press of the alternate flaps position switch, the LE flaps extend and LE slats extend to FULL EXTEND position via the standby system.

All this makes the autoslats (which move LE slats from EXTEND to FULL EXTEND) moot, as the LE devices are already fully extended.

[repulo]

The PTU control valve will move to the closed position when the alternate flaps switch is in the armed position. Also, the PTU needs the flaps not to be in the up position to run, so the alternate flaps switch had to be used. As stated above, that already sent the LE devices to the full extend position.
Should the flaps not be up and sys B press is lost, the PTU would run. But it only supplies pressure to the LE devices. Now the LE devices are controlled by the TE flaps and they will not move. So here we are again at the alt. flaps switch switching of the PTU..

[Banana Joe]

Alright, I did not find anywhere in the FCOM that the PTU control valve will move to the closed position when the alternate flaps switch is moved to the armed position. Now it makes sense. In the sceario given above with only system A available then it will be a F15 landing with no LED and 5 knots will be added to Vref F15.

[repulo]

Banana,
the question about the PTU is as old as the 737, that was the reason I had a look into the AMM, where the condition is described.
Concerning your scenario, I think it’s rather VRef 15+15. You will have to cater for the LE devices not being extended and for the TE flaps being at 15 degrees.

[Banana Joe]

My QRH says to use Vref 15+5 and to limit bank angle to 15 degrees when below flaps up maneuvering speed.

[repulo]

With no LE devices extended? You’re sure?

[Banana Joe]

Yes, the LEADING EDGE FLAPS TRANSIT NNC checklist says to plan a Flaps 15 landing, set Vref 15 + 5 knots and limit bank angle to 15° when airspeed is less than flaps up maneuvering speed.

EDIT
I should have clarified that I was talking about a 737CL. I am currently not flying the NG but on the NG it's Vref 15 + 15 knots.

[repulo]

That makes sense:)

[lederhosen]

I have actually had a loss of system B in an NG with complete fluid loss. The leading edge devices and flaps all extended and the flaps 15 landing was a complete non event. Quite often things in the sim are more difficult than in real life which is probably how it should be.