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windshearDK

Deactivating HYD SYS A for pushback

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Quick question, the manual recommends turning off system A to prevent damage to the nose gear on pushback.My question is, if this is recommended for both the electronic and engine driven hydraulic pumps? Oh by the way, I love love love love the NGX!!!

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Yes, you turn off both so you can start both engines while pushing. Although, IIRC some airlines simply pull the nose gear steering pin which disconnects the hydraulic system from NWS, so they just pressurize both systems before push.

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Quick question, the manual recommends turning off system A to prevent damage to the nose gear on pushback.My question is, if this is recommended for both the electronic and engine driven hydraulic pumps?
If you are starting both engines on the pushback, then yes - both A pumps off.

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My initial thought was: Of course only the "elec hyd", but then I thought, hey I am starting the engines on push...!! Ok will deactivate both the alpha elec and eng driven pumpscool.png

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My initial thought was: Of course only the "elec hyd", but then I thought, hey I am starting the engines on push...!! Ok will deactivate both the alpha elec and eng driven pumpscool.png
The reason being that nose wheel steering is actuated by HYD A. If there's pressure in the system, some pushback trucks wouldn't be able to steer the airplane. So, both pumps for HYD A off.

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It's not neseccary to switch OFF Hyd A for pushback when the nose gear steering lockout pin is installed (see FCOM 1 page NP.21.30) and most pushback drivers install's this pin during pushback. On the nose gear there is a lockout lever to block hyd A pressure to the steering actuator during pushback.

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It's not neseccary to switch OFF Hyd A for pushback when the nose gear steering lockout pin is installed (see FCOM 1 page NP.21.30) and most pushback drivers install's this pin during pushback. On the nose gear there is a lockout lever to block hyd A pressure to the steering actuator during pushback.
Correct...B pumps off was required in the 100 and 200 series...from 300 series on bypass pins are used....

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There isn't such thing as not all of the A system. It's either pressurized or not. There are more complex situations but, on normal operation, it's either presurized or not pressurized.You have to turn off both sources of pressure for HYD A. Otherwise, as soon as engine 1 starts, the engine driven pump will presurize the system. If on alternate nose wheel steering, then then nose wheel steering is actuated by HYD B.

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There isn't such thing as not all of the A system. It's either pressurized or not. There are more complex situations but, on normal operation, it's either presurized or not pressurized.You have to turn off both sources of pressure for HYD A. Otherwise, as soon as engine 1 starts, the engine driven pump will presurize the system. If on alternate nose wheel steering, then then nose wheel steering is actuated by HYD B.
The Hydraulic System's is during pushback normally pressurized, Before the pushback driver start's with the pushback he install a lockout pin in the Nose Gear steering Lockout lever, This Lockout lever is pulled to block the Hydraulic pressure from Hyd A to the Steering actuator, Hyd B pressure is normally not used for steering. Alternate nose wheel steering is not automatic you need to switch it over with the Nose Wheel steering switch located on the Capt instrument panel underneath the Capt's clock.

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The Hydraulic System's is during pushback normally pressurized, Before the pushback driver start's with the pushback he install a lockout pin in the Nose Gear steering Lockout lever, This Lockout lever is pulled to block the Hydraulic pressure from Hyd A to the Steering actuator, Hyd B pressure is normally not used for steering. Alternate nose wheel steering is not automatic you need to switch it over with the Nose Wheel steering switch located on the Capt instrument panel underneath the Capt's clock.
Thanks. So is leaving HYD A pump switches off for "just in case"?

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You can leave Hyd sys A OFF because it's not needed for pushback. But that is not my advice, Because than you have to Start Engine #1 after pushback, Do not switch OFF the Engine Driven Pump (EDP) if it;s not a emergency, you can damage the EDP solenoid. Most airline's are starting both engine's during pushback, Switching OFF the EDP is not a normal procedure in the checklist for engine start.So my advice is to switch ON the Hydraulic pumps during pushback and start all engine's and get going !!!!.

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You can leave Hyd sys A OFF because it's not needed for pushback. But that is not my advice, Because than you have to Start Engine #1 after pushback, Do not switch OFF the Engine Driven Pump (EDP) if it;s not a emergency, you can damage the EDP solenoid. Most airline's are starting both engine's during pushback, Switching OFF the EDP is not a normal procedure in the checklist for engine start.So my advice is to switch ON the Hydraulic pumps during pushback and start all engine's and get going !!!!.
...but. As a flight student I did many MANY jumpseat hours, and they always had both ELEC and EDP off on system A during pushback (pumps A off and pumps B on was in fact part of the before pushback flow). During pushback they'd start both engines up. After pushback, they would turn on both pumps on HYD A (engine1 being already on). Talking about Air Europa and the (now inexistant) Futura and i saw this both on -400s and -800s. Also, if my memory doesn't fail, I also saw this many years ago in Lan Chile's 737-200s. Also, and noticing you are an engineer, I have been told many times that fuel pumps don't need to be on for the APU, yet in both of these airlines I always saw a pump switched on for the APU. Are these pilot's "superstitions"?

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If they are not using the lockout pin then is it to the Pilot In Command to switch OFF hydraulic A switches (ELEC and EDP switches).And for the APU, yes it's not necessart to switch ON the Fuel pumps. But it's better for the APU performance to switch the fuel pump(s) ON.

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If they are not using the lockout pin then is it to the Pilot In Command to switch OFF hydraulic A switches (ELEC and EDP switches).
So how is this communicated in RW? Sounds like an area prone for mistakes and mishaps. If I was ground crew I would always insert the pins whether HYD A is OFF or ON. Cheers,

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If they are not using the lockout pin then is it to the Pilot In Command to switch OFF hydraulic A switches (ELEC and EDP switches).And for the APU, yes it's not necessart to switch ON the Fuel pumps. But it's better for the APU performance to switch the fuel pump(s) ON.
Ok then everything makes sense. Also, refresh my memory on something please. I remember something about a 1 minute cooldown for a system I can't rember. Could it be that one pump remained on for 1 minutes after shut down for off-load cooling? I really cant remember anything else... just "one minute cooldown". I know the pumps' refrigeration fluid is the fuel itself, am I right?
So how is this communicated in RW? Sounds like an area prone for mistakes and mishaps. If I was ground crew I would always insert the pins whether HYD A is OFF or ON. Cheers,
I think it works both ways. Ground staff will always use the pin, and tech crew will always keep no pressure in A until they have a visual of the pin.

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The èreferred way is to use all precautions, bypass pin AND A pumps off. The reason is human factor. Someone can miss to install the pin, someone can switch the steering on alternate for example. The danger in failing to deactivate steering could cause damage to the steering components, its not a problem of the tractor, the steering will break if hydraulics locks it.737 classic uses an hydraulic heat excanger inside the fuel tank to cool the hydraulic fluid. It is adviced to not use hydraulics when fuel is less than 760Kg in each wing. I must check on NGs.

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I think the cool-down period is for the APU and when you should turn on APU bleed air. shouldnt you wait 1 min before using APU bleed air?

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1 mnute is needed to let stabilize all APU components, to warm them up at the right working temperature. The cooldown of the apu is done automatically.

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I wrote the question based on the bulletin written in the normal crew flow in the FCOM.There it was advised that you turn off "hyd sys A" as the turning of the nose gear could damage it, if sys a was active. I did a successful push and start with sys a on and off, so I do not know which is the correct procedure. My question arose, because the manual simply refers to "sys A", and not sys A elec or eng. Does this help?

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I wrote the question based on the bulletin written in the normal crew flow in the FCOM.There it was advised that you turn off "hyd sys A" as the turning of the nose gear could damage it, if sys a was active. I did a successful push and start with sys a on and off, so I do not know which is the correct procedure. My question arose, because the manual simply refers to "sys A", and not sys A elec or eng. Does this help?
Both pumps off, ELEC and ENG. Check the schematics of what each does (pretty much the same: Pressurize HYD A)
The cooldown of the apu is done automatically.
I remember that from the 777.

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Simply when a manual tells to turn off a system, such as an HYD system, the pressure in that system must be 0. Look at the indications. Both EMDP and EDP provide pressure for the same system, so the logic is: turn both off, or if I am sure that engine will not run untill push is finished, I turn off only the ELEC (and confirm with the pressure indication that pressure is 0)

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Simply when a manual tells to turn off a system, such as an HYD system, the pressure in that system must be 0. Look at the indications. Both EMDP and EDP provide pressure for the same system, so the logic is: turn both off, or if I am sure that engine will not run untill push is finished, I turn off only the ELEC (and confirm with the pressure indication that pressure is 0)
If the pressure was 0 we'd all be a little dead...

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Not absolute pressure, but hyd sys relative pressure as seen on the gauges.wink.pngThe hyd pressure will be around 30/45 psi as the system is pre-pressurized to prevent pump cavitation. Air to do it comes from the bleeds.737 can fly w/out hyd pressure, only the rudder needs a stby system as it is fully hydraulic.On the last test I did the Stby rudder is not correctly simulated as the rudder didn't move with flight controls in STBY RUD position and no hyd system pressurized. Also other flight controls won't work. The only flight control surfaces moving with stby system are the LE devices. Must check for reverser stby operation.

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