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Ashatsea

Switching from APU to Ground Power

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I was on a 737 as a passenger a couple of weeks ago and I was trying to correlate the dips in power (On the ground at the gate) with different equipment such as switching from Ground Power to the APU and so on. When a aircraft pulls up to the gate and the Marshall gives the pilot the wand across his neck which I assume means secure the engines does the pilot wait for ground power to be hooked up and then switch over before securing his engines or does he turn on his APU after landing then kill the engines at the gate and wait to secure the APU when ground power is hooked up or does the pilot secure the engines/APU and stay on battery until the ground crew can hook up the power? Are there procedures standard throughout all aircraft or does it vary from company to company.

 

Last question do most airports provide ground power through a cord under the jet way meaning the Power cart is physically built into the moving bridge or are portable Ground power units still available?

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Of course the procedures vary from company to company, airport to airport.

 

Here's the general usage:

 

After landing, APU On

At gate, APU Gens ON, APU Air on,

Engines shutdown,

Ground Power plugged in, the signal is given, and ground power is applied.

At this point it's up to the crew if they want to keep the APU on for air conditioning/heat.

 

At airports a little more anal about APU usage:

 

After landing APU stays off

As the aircraft approaches the gate, typically shut down one engine.

Ground power is plugged in, and applied.

Shut down other engine.

Secure aircraft.

 

 

Basically you have the right idea. You want to ensure you have the other power source availible before transferring to it. Then you can get rid of the previous power source.

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Most of the time at the company I work for when the airplane arrives at the gate, the GPU is connected and the Crew switch over from IDG'(s) to ground power and than shutdown the engine'(s). When switching to a other power sources it gives a power dip as you noticed through the cabin lights. The APU will not be started during arrival of the airplane. Also now day's airlines also shutdown engine 2 after landing and taxi in with engine 1.

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9 times out of 10 we start the APU on the taxi back and put it on the buses, which is the last item of the after landing flow. If conditions permit we may taxi in with engine 2 shut down, but it's our SOP to always shut number 2 down as we pull onto the stand, before we've actually stopped... mainly for safety, as most of the ground equipment is on that side of the plane.

 

Obviously as said above this varies with equipment available and airline/airport procedures. After we've shut down and get ground power we usually turn off the APU and keep it off until we need it for starting again. We only have it on whilst on the stand obviously if there's no ground power, and if the temperature is below 5 or above 25 degrees. Wouldn't want the passengers complaining would we...

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Ash

 

A little off-topic (sorry guys), but this link shows you the marshalling signals which you may be interested in and certainly clarifies your comment on the 'cut engines', amongst others.

 

I figure the other points have already been covered.

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Last question do most airports provide ground power through a cord under the jet way meaning the Power cart is physically built into the moving bridge or are portable Ground power units still available?

 

At ORD they usually use a cord that is attached to the jetway and the only time I see them bring out the car is if the jetway power isn't working. Also with the MD-80 they do use the APU on the ground a lot as the ground air isn't capable of keeping the jet very cool since the hose is attached all the way back, during the summer the APU seems to be running during most of the turnaround mainly to keep up with the air conditioning.

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