Originally Posted by
wrench1
..Ever wonder why a twin engine helicopter has an aircraft based 5 min limit with both engines on line, but when OEI the limit drops to 2 ½ minutes per engine? Me too.
Your point is escaping me a little bit there. Twin engine 5 minute takeoff limit is so you don't rip the guts out of the helicopter MGB/TGB/IGB and driveshafts. The OEI limits are so you don't rip the guts out of the engine.
And this talk about reason for 5 minute take-off limit in R44, or whatever helicopter. I mean, who cares whether it's for engine warranty or engine TBO or MGB or anything else? It's not something the pilot needs to know. The examiner might ask you, but he can't fail you for not knowing...he might fail you for not knowing the said limit as published in the RFM, but he can't fail you for not knowing why or how or whatever speculation not published in the RFM.
The 5 minute take-off power limit is just that. From the hover, commencing takeoff pull up to that power limit, 5 minutes starts, once clear of obstacles, set MCP or climb or cruise power. If after 5 minutes you still aren't clear of obstacles - and I'd be wondering why that might be the case - you keep pulling that 5 minute power for as long as you need to. And if that amount of power 'aint enough to avoid bumping into something in the take off path, you use all the power the engine can give you, for as long as you need it. And at the end of the flight make an appropriate entry in the technical log.
You can't be using 5 minute take off power in the cruise just to go fast, or to climb fast, back it off at the 5 minute mark, and then grab another 5 minutes worth of take-off power to go fast. That is not what 5 minute take-off power is all about...and besides, some helicopters have power limits based on airspeed.