In the event of a port engine failure during take-off, the landing gear would not be able to retract as there is no auxiliary motorpump in the green hydraulic system on an A320.
An additional motorpump may be heavier than a PTU however, and complex trade studies may favor one option or the other, depending on which failure cases are considered and how important weight is in the trade-off.
On yet other aircraft, the function of a bi-directional reversible PTU can be accomplished with two uni-directional PTUs installed side-by-side arranged in opposite orientations to each other.
In yet further representations, a bent-axis fixed-displacement motor/pump can be mated with a straight-axis variable displacement motorpump, as in the case of the Airbus A320 PTU.
Consequently, a PTU accelerates very rapidly under the delta-P induced load, and then stops just as suddenly once the pressure equalizes.
Passengers who have flown on the Airbus A320 will frequently hear the PTU 'barking dog', generally when only one engine is running, or when the yellow system electric motorpump is the only active hydraulic power source; the PTU is mechanically activated in these cases.
Very rarely is it heard in flight unless a momentary power deficit is present when retracting the gear, or a hydraulic fault has occurred.
However, solenoid energized shut-off valves can isolate the PTU via a push-button switch (pb/sw) in the cockpit, but this feature is rarely used.