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View Full Version : "A" pumps off for pushback -737MAX & NG


lowspeedaluminium
20th Jul 2018, 16:17
I'm curious as to how many operators out there turn the Hyd A pumps 'off' for pushback on the 737NG and MAX.
At my previous operator (737-800) we had all Elec and ED hyd pumps 'on' for pushback -- given the nose gear steering lockout pin, having the pumps on seemd a non-issue (also Boeing procedures).
At my current outfit (737Max), the sop is to turn both 'A' pumps off for pushback.
thoughts on this? Unnecessary? Surely this would result in excessive wear on the EDP?
This outfit used to fly the -200 many years ago, perhaps just cut and pasted procedures. Thoughts?

FlyingStone
20th Jul 2018, 16:22
Why would there be any wear on the EDP itself? The EDP keeps running regardless of whether the EDP switch is ON or OFF...

The "A pumps off" just adds an extra safety barrier in place, might be useful if operating to places with unreliable ground crew. Not in use at my airline currently...

wiedehopf
23rd Jul 2018, 12:07
Electric demand increases the load on the APU decreasing available bleed air power.
I would guess a electrically powered hydraulic pump qualifies as a large electric load?

A and C
23rd Jul 2018, 13:30
This is a throwback to the early B737-200 that had no steering lock out pin fitted to the nose leg , it’s all part of aviation’s inertia and a totally pointless procedure on the -800.

ACMS
23rd Jul 2018, 14:53
APU prioritizes Elec over Bleed, so of the APU is struggling offload Elec load, more duct pressure less chance of a hot start. We did that 25 years ago on 737ís.

Uplinker
6th Aug 2018, 09:21
Without knowing the circumstances, my guess is that the operator in question had a close shave when a steering pin was either not fitted or malfunctioned, and the steering hydraulics became pressurised. So the SOP was probably changed to ensure the steering hydraulics remained unpressurised until the push crew had removed the tow-bar.

At one time we used to switch the nose-wheel steering off during pushback (Airbus), but in later years we we left it on, I think there was a software update.