"Nose Wheel Steering Bypass pin MUST be installed or BOTH System A HYD Pumps must be off when towbar etc connected. This is job of Engineer to both fit and remove the pin."As it is on all Boeings... I understand this wholeheartedly, guys. I even had to explain this a few weeks ago to a rookie 747-400 pilot doing a walkaround at the gate when he asked me if the pin behind the nosegear should still be inserted (i.e. the nosewheel steering pin). I was just having problems understanding why it should make any difference if you transferred electrical power. The engine pumps are not going to do anything differently if you change electrical power (as they are mechanically driven) and 3000' psi is already there, so having the elec pumps go off and come back on line due to a power glitch is not going to do anything extra.If you switch on the elec pumps alone, I can see how things like the control surfaces and the column might kick as they get pressure, but if you already have pressure there from engine pumps, I can't imagine them kicking again. Perhaps next time you have the opportunity, you can manually switch off the elec pumps when the engine pumps are running... and see if anything happens to the steering/control column when you switch them back on again? (I don't usually get to play with this sort of thing ;-)).Thanks!Cheers.Ian R.P.S. (Edit) Quick quiz.... Will setting rudder trim, move the nosewheel? :-)

Ian,If I read an earlier response correctly, the issue is that the valve that keeps the engine driven hydraullic pump from pressurizing the system is electric and the transient from bringing the generator online could cause it to open. Thus the "extra" to which you refer is suddenly allowing that 3000 psi to make it out to the NWS.cz

>P.S. (Edit) Quick quiz.... Will setting rudder trim, move the>nosewheel? :-)As far as i know it shouldn't.BRClaudio

"the issue is that the valve that keeps the engine driven hydraullic pump from pressurizing the system is electric and the transient from bringing the generator online could cause it to open."Sounded good to me for a moment, there, Richard, but then I remembered that the electric side of the Engine Driven Pump is such that if you lose electrical power when the pump switch is in the ON position, nothing happens to the engine hydraulic system. You need to manually/physically move the switch on the overhead panel to OFF to get any power to the pump control solenoid. Power glitches will only affect the pump state if the switch is in the OFF position.Cheers.Ian.

We have reached a level beyond me here, this is bordering on engineer speak which I really don't understand. All I know is we aren't supposed to touch anything till we see the man with the pin waving. Thats all I need to know really, that when I do my thing and then it's off to sunny spain.... again.... and again (oh hard life eh!)Ian, when you go to Engineers school do they issue you with a magic wand or a magic percussion adjuster (hammer).Kris

Yeah Kris, must be a real hard life - you can change with me anytime you like...;-) When you're at FL350 headed for sunny Spain I'm locked up in a server-room fixing computers 9-5

Quick Quiz Answer! One would hope not, The nose wheel steering is affected by the pedals but shouldn't move with rudder trim or when you check it you'd send servisair start crew members flying into the next county! Kris

Hi Ian!I was the one who said that earlier in this thread.According to my notes, aswell as a few simrides (note, aircraft involved was a Lufthansa 737-Classic) loosing power while the engine is running with the hyd. switch in the off position will pressurize the hyd system! This is to make sure, in case of a full electric power, you still have hydraulics. I believe the bus in question would be the right AC bus, so I _guess_ you could connect the left generator with the APU still supplying the right AC Bus

"One would hope not, The nose wheel steering is affected by the pedals but shouldn't move with rudder trim or when you check it you'd send servisair start crew members flying into the next county! "Then here's a few hints in the form of more questions.... What happens when you trim the rudder? Do the rudder pedals move?How do you keep the aircraft on the centerline when you've started your takeoff roll? (Before your rudder surface becomes effective).Unless a 737 is totally unlike a 767 and 747 in this department, I could be barking up the wrong tree.Cheers.Ian.P.S. Has Servisair sent you a bill yet for their return ticket from the next county? ;-)

"We have reached a level beyond me here, this is bordering on engineer speak which I really don't understand."I know how you feel... I've been trying to come up with an simple analogy to help explain the control circuit and haven't been having much luck.Anyway, in the diagram below, there is a simple light circuit... except it has been wired back to front. When you put the switch to ON, the light goes out. The switch represents the engine hydraulic switch.http://members.ozemail.com.au/~b744er/737NG/LightCircuit.gifLet's say that when the light is illuminated, the hydraulics are shut off and that when the light is not illuminated, the hydraulics are on.OK.... If you have the light switch set to ON, the light will not be illuminated... and the hydraulics will be powered. What happens if you lose electrical power (due to a power glitch) in this configuration? What will happen to the light? The light will remain off... so hydraulics will remain powered.This is basically how the real aircraft works (except the light bulb is a shutoff solenoid).Something to think about when you're laying back on the beach in sunny Spain :-)Cheers.Ian.P.S. BTW, I was issued with "Defect Not Confirmed on Ground Check" magic pen when I left Engineering school ;-)

Thanks, Mark... Basically, the problem is only generated if the Engine Hydraulic Switch is OFF when you hit the GEN switch.On airlines which have the hydraulic pumps off for engine start as normal procedure, this could create a danger for engineers. On airlines which do have the engine hydraulic switches ON, it won't be a problem.Procedures are often simplified to cover a range of aircraft with different options (to make it easy on the pilots ...and safer for the engineers). Unfortunately, the why's and wherefore's of these rules and regulations are lost on the guys who are relatively new to the business. As an engineer, you have to know the why's and wherefore's.Cheers.Ian.

I've tried to follow this, but some of the terminology is a bit hard to swallow. To sum it up, don't turn on the generators until everybodys done playing ring-around-the-rosie with your nosegear?

"To sum it up, don't turn on the generators until everybody's done playing ring-around-the-rosie with your nosegear?"Basically, you do what your Chief Pilots/Procedures manuals tell you to do, Christian ;-)If you haven't got a Chief Pilot, your engines are running and providing you with hydraulic presssure, as far as I can see, it shouldn't matter what you do with the gen switches. But, as always, I stand to be corrected. It would make more sense to me to not have any hydraulics on before the engineers are clear of the aircraft, but then this is not practicable on some aircraft types).Cheers.Ian.

I'll also try to sum it up:this thread is only then of concern, IF(a) your aircraft does NOT have nose wheel steering bypass pins(:( you start your engines while you are being pushed backIf this is the case, the make sure all hydraulic pump switches for Hyd System A are off and that you first engage the generators when you get a signal from your ground crew that they've cleared the aircraft.Regards,Mark