Originally Posted by
wrench1
gulli:
We're drifting off topic...
Why is a twin OEI limited of 2.5 mins exactly half the aircraft 5 min limit? Why not 3 mins or 1.5 mins?
If a twin OEI limit is higher, why not apply the same limit to the same core engine installed in a single engine aircraft?
Yeah we are drifting, but for the benefit of those who might not be familiar with twin engine helicopter limitations:
Let's say for example, in twin engine helicopter the 5 minute take-off power limit is 100% TQ. At that power setting the MGB sees 2x100%TQ = 200% TQ. The limiting factor is due to the MGB. Now let's say you are OEI in that same helicopter and you are operating at the OEI limit of 127% TQ. The MGB sees 127% TQ: remember, it can do 200% TQ for 5 minutes. Easy, no sweat at all for the MGB at any OEI limit. But that engine sure is getting hot pumping out 127% TQ for (up to) 5 minutes.
Which is why I said earlier, the 5 minute take-off limit is stop you ripping the guts out of the MGB/IGB/TGB and driveshafts, the OEI limits are to stop you ripping the guts out of the engines.
This is a simple explanation, the actual situation is slightly more complex because in some twins there are things such as 30 second OEI power limit, dual engine soft limiting, and dual engine blow away power, and combined TQ limits when one engine is better than the other, blah blah blah, but the principal is the same.
And finally, an angle to your question is why is the 2.5 min OEI limit half the 5 minute twin engine minute limit? It is only half in time, not half (actually double) in power. Why it is half in time I don't know, and I don't care, I'm only the pilot and I only know what is written in the RFM. And if that examiner asked me the same question that was raised at the opening of this topic, my answer would be "don't know, don't care". Sure, he is entitled to ask the question, but I don't need to know the answer.