Originally Posted by
krypton_john
What's the AC for? Is it so you can plug in a kettle and a microwave to warm up those inflight meals?
Krypton, As far as RAF HC1s were concerned, not much chance for in flight catering at 140 kts and 50 feet agl...
The glass windscreen heaters were AC (earlier aircraft we flew had unheated, plastic windscreens). Attitude and stability system gyro power was another important one.
The RAF Pumas didn't have the more common engine driven, starter/DC generators. Instead it had separate AC generators driven from the main rotor gearbox. The advantage of that is you don't instantly lose a generator if an engine stops.
However, the disadvantage was that both alternators would instantly drop off line if the rotor rpm (Nr) was allowed to "droop" too low (initially 220 rpm; 265 +/- 7 being the normally governed Nr). This was a factor in a fatal accident in Belize in 1976. The aircraft lost an engine lifting from a jungle helipad at night. The remaining engine couldn't maintain rotor rpm, the alternators dropped off line, which would have resulted in loss of aircraft attitude stability. The aircraft drifted back over the helipad, crashed into the jungle and burned out, killing all on board. The system was subsequently modified to allow 2 seconds at 240 Nr to allow for a transient droop. I can't say if this was a real advantage, thankfully the Puma engines proved to be totally reliable in my ten years or so flying them.
There were two transformer/rectifier units (TRUs) which converted 26V AC to 24V DC for most other services. They were situated in the "broom cupboard" behind the pilot's seat.
I did once experience a TRU overheat/fire, which was quite interesting for a while because of the fumes coming from it. We were flying at low level (would have been much more serious had we been IMC) and were able to quickly land in a field in Wales; whereupon I was briefly mistaken for the English rugby captain by the farmer and his family! I still remember the subsequent looks of disappointment on their faces....
Edit: The photo posted above is obviously a French machine, probably military. The RAF HC1s didn't have quite the same layout..and words in English!