Two questions in this thread really....

First there is a sticker on the rear wall of the cockpit above the APU panel that says
"Caution clear reversers before starting APU"
What does this mean exactly?

 

Second, if the plane is plugged in to External Power and External Bleed Air, why do I need to start up and use the APU? The thing is bloody noisy.

 

If it matters, this relates to the CS727 btw.

For your first question, I would assume that it means the reversers need to be stowed on the engines. Since they deflect airflow to slow down, they may affect airflow going into the APU, whether it be not allowing air to easily flow into the APU or putting hot exhaust air into the APU. Therefore, you need to make sure your reversers are stowed after landing before turning it on.

 

As for the second question, some aircraft require both to fully power electronics and provide a/c for the cabin. Whether that is the case on the 727 I don't know. But typically on any airliner, if you are getting push back, you need to start the APU before disconnecting from ground power, that way when you push you won't lose electrical power, and you'll have bleed air ready to start your engines. If you're starting at the gate, you can use ground power and an air starter unit to start up, depending upon if that's allowed per the manufacturer or by SOP's.

Adding to Kyle's points, the engine start can happen with any kind of electrical and pneumatic power source being available. So if you would be allowed to start one engine at the gate, you could disconnect all external sources, get pushed back and start the rest without the APU being necessary.

 

As for the reversers, here's a pdf on the APU's location. As far as I can tell, there might not be a problem with the reversers blocking some inlet or exhaust, but those units might come alive as you power up the APU. Maybe it's the same as with making sure your flap lever corresponds to the actual flap position. You don't want the arrangement to start moving 'uncommanded' when you power up your plane.

 

So my guess would be that they want to avoid situations like these. Warning, gore. Poor Steve. :unsure:

This should also go into the 727 section over at CS, along with your two recent posts. Capt Lou over there has many years flying the 72s for TWA. He even has a thread dedicated to his stories there. He knows this airplane intimately and hopefully he will answer these questions.