PTU
The purpose of the PTU is to provide automatic back-up power for the slats to move from extend to full extend if the EDP in the B system has a low reading, the flap handle is in “ not up” or further than 10 units and the aircraft is in the air. The pressure is used if an auto slat command is given.
So in summary;
The PTU control valve supplies A pressure to the motor side of the PTU if all these conditions are met:
· The aircraft is in the air,
· Flaps not up or further than 10 units, (flaps are in the extend)
· The pressure from the EDP in the B system is 2350 psi or lower for > 0.5 sec.
If the pressure from the EDP B is returning to [FONT='Arial','sans-serif']values above 2350 psi it will not close the PTU control valve since it is latched after 0.5 sec.[/font]
The PTU pressure cannot be used for retraction of the slats since the flap handle is than selected in up, which make the PTU to stop. (see the conditions above)
[FONT='Arial','sans-serif']The Landing Gear Transfer Valve[/FONT] has nothing to do with the PTU system. Altough it has to be mentioned that this valve will be closed if an auto slat command is given [FONT='Arial','sans-serif']simultaneously[/FONT]
with a PTU control valve open signal, the LGTV will be closed to reduce the power drop during the landing gear up selection in the “A”system. A pressure drop in the “A” system affects the speed of the PTU motor and so the output.
frits, technical instructor 737.