Why no PTU on the 767?

[stilton]

I am sure it is obvious but I don't remember why this was used on the 757 and not the 76.

[None]

I'm no expert, but here's my understanding concerning the 757 PTU.
The short story (as I understand it):
High-interest items are on the 757 left hydraulic system. The 757 PTU backs-up (replaces) the left engine-driven pump (EDP) when its output is insufficient, thereby ensuring the operation of the high-interest items.
The 767 has the high-interest items on the Center Hydraulic system. The two AC motor pumps (ACMP) are backed-up by the Air Driven Pump (ADP). Therefore it does not need a PTU for a back-up.
The longer story:
The 757 left hydraulics, which has flaps/slats/gear/steer, utilizes an EDP, (primary), and a supplementary ACMP. If you shutdown the left engine, and therefore the left EDP, the PTU will come on-line to assist the ACMP to operate these high-interest items. The 767 center system does not use an EDP, but rather two ACMPs. As the ADP backs-up the ACMPs, a PTU is not necessary for the 767.
Having said that, the 767 does in fact have a PTU. It is used as a back-up for stab trim.
POWER TRANSFER UNIT (757)
Control
Loss of left Engine RPM (below 51% N2) or a drop in Engine Driven Pump
(EDP) output pressure below 1200psi, with the right Engine running, will
complete a circuit to open the PTU Control Valve. If pressure is not available to the PTU CONTROL PRESSURE switch within three seconds, the PTU on-relay will energize closing the control valve. With the left engine running and with high EDP pressure, output power is available to the close side of the control valve. Operation of the PTU switch on the overhead panel (P5) will provide power to open the Control Valve.
Indication
The PTU will display as left system pressure on EICAS with both left pumps off.

[haughtney1]

ADP?...do you mean the RAT?:}

[stilton]

Thanks for the good explanation. I had not thought of the ADP as a back up to centre hydraulics on the 767 in the same manner as the PTU is to the left system on the 75 but more of a supplement.

I assume you mean the PES (pitch enhancement system) for stabilizer trim on the right system by alluding to a PTU on the 76?

[None]

I assume you mean the PES (pitch enhancement system) for stabilizer trim on the right system by alluding to a PTU on the 76?
Precisely.
>haughtney1 ADP?...do you mean the RAT?
No.

[stilton]

Thanks again for your insight.

[Bolty McBolt]

no PTU on a 767?

Isnt there a PTU that lives in the forward horizontal stab compartment in the ceiling area and is used as a backup supply for running Stab trim in a loss of one/two Hydraulic systems (not sure of config)

I am sure I will be corrected if I am wrong

[Swedish Steve]

no PTU on a 767?
Isnt there a PTU that lives in the forward horizontal stab compartment in the ceiling area and is used as a backup supply for running Stab trim in a loss of one/two Hydraulic systems (not sure of config)
I am sure I will be corrected if I am wrong
Yes this is the Pitch Enhancement System PTU. With loss of Left and Centre system pressure it transfers Right system pressure to the Left Stab Trim Motor. AMM 29-22-00

[Bolty McBolt]

Thanx Swedish Steve
I knew the 767 had DC 10 technoligy...PTU

[stilton]

Not to belabour the point, but why does the 767 have the PES and not the 75

Any ideas?

[Bolty McBolt]

Stilton said... why does the 767 have the PES and not the 75

Stilton....No PES fitted to early model 767 ... nor was it retofitted to models with out.

When I did my training on the 767, I was told the difference between the 757 and 767 is the Airbourne Vibration Moniture Computor was in a different rack in the MEC but most boxes are interchangable and you step up into 767 cockpit and Down into 757 and thats it...



Bolty

[None]

Not to belabour the point, but why does the 767 have the PES and not the 75
Any ideas?
Good question.
Here's one theory.
When you and I fly these jets, they are basically the same, or at least very similar. To the maintenance and engineers, they are two different airplanes.
The 757’s are assembled in Renton, WA and the 767’s are put together in Everett. The design teams are two different groups, and perhaps they did not confer on every aspect of the jets.
Apparently the 767 design team thought that pitch control was a critical item, and so designed the PES as a redundant system.
The 757 does not have a PES for pitch control, but does have the PTU to operate the “flaps, slats, gear, and steer” in the event of a Left Engine failure or in the event of a failure of the Left Engine-driven hydraulic pump.
Maybe one team thought the pitch control needed redundancy while the other team thought that the “big four” needed redundancy.
There may be other theories out there as well.