tom24,
Tell your mate he's wrong! Why would you need extra electrical power for t/o? you need max EPR at t/o hence the need to switch off compressor bleed air systems, such as anti-ice (most of the time!), Air conditioning, etc. The APU provides electrical pwr until it is shut down shortly after departure.
Tightslot has the actual reasons correct.
Also- leaving the window blinds open for t/o so that a/c is easier to be seen? -Bol**cks! Maybe worth a try if you have a complete strobe & nav light failure perhaps?! It's done so that pax can alert the crew if they see fire, not mandatory, but some airlines have adopted this procedure since the Kegworth disaster.
Tri
