B737 (NG) PTU

[fireflybob]

Can somebody please remind me of the principle of operation and purpose of the Power Transfer Unit on the B 737 NG ?

Thanks for any help.

[BOAC]

The Classic as well - if B system engine driven pump/number 2 engine fails, the PTU uses A system pressure to power a pump which pressurises B system fluid for flap/slat opertaion. Flaps extended but less than 15 and airborne only.