What is the source of emergency power for a 727? I've found a couple of sources that state the 727's APU cannot be started while flying, as it draws air from the wheel well. 


  It drew air from inside the wheel well, so it was only for use on the ground. 


https://aviation.stackexchange.com/a/37715/34744


  The APU on the Boeing 727 can be used for ground operation only.


https://www.boeing-727.com/Data/systems/infoapu.html

Additionally, the battery on the 727 doesn't power all of the systems on the "essential bus", and presumably cannot start the engines. 

http://cyberpilot.tripod.com/electrical_system.html

So what happens if you lose all engine power in a 727? How can the engines be restarted?
According this rather detailed page on the 727's engines:


  To restart in flight, an "air start" may be attempted. The start
  switch should be placed in the FLIGHT START position which will arm
  high energy ignition. The starter valve will not open. Ram air
  entering the engine is sufficient for start if the aircraft's speed is
  above 150 knots. When the start lever is moved to IDLE, fuel is
  introduced and the igniters are activated.