Today I was watching a documentary on the history of helicopters, at one point a pilot was going through the checklist and starting up the helicopter when, briefly, I heard that distinctive alarm that rings in every single helicopter crash on the movies.
You know which one, chopper hit by bullets, giant mutant, mountain top, overweight, tree strike, etc, etc… wooopwooopwoop you are going down!
So what is it actually for in reality? My guess would be low rotor speed, but just guessing.
Altitude loss and excessive closure to terrain.
IANAHP, just a guy who uses Google.
Depending on the model and age of the heli, it’s probably the low rotor speed warning. Mainly the most deadly thing that can go wrong. You can land without an engine, without a tail rotor, but if your rotors aren’t turning, you’re a stone. You test the alarm before flight.
Naturally, movies never get it right. I’ve seen MD500s and 600s in “movie crash” mode where all the caution/warning lights are flashing. Many of the lights in the 500 are for things like impending air filter bypass, and transmission chips and engine anti-ice. Half of them are advisory, not warnings. So it looks silly when the movie implies impending doom by having all the dash lights flash. The important ones are engine out and low rotor, and fire, if the aircraft has fire detection (it wasn’t always required).
The 600N doesn’t have a woop alarm at all, I believe. Just voice warning. Fun fact: When we first made the 600N, the company supplying the voice warning was in Texas, and they used an employee to make the warnings. Trouble is, in a Texas drawl, POWER and FIRE kind of sound the same. Pahr, Fahr. We had to get them to find a midwestener to record them!
Another movie heli nitpick: in movies like Broken Arrow, the heli is knocked out of the sky by the EMP of the exploding nuclear bomb. You know what would actually happen? Nothing. The engines are all hydromechanical. There aren’t any electronics to affect. In the 600N, which is FADEC controlled, all the important engine electronics are HERF shielded, and should they go out, there is manual backup. You could fly all the way home with no problem.
In my (moderate amount of helicopter) experience, “Whoop-whoop-whoop” is low main rotor speed, what rotorheads call “Nr”. Low Nr is a Very Bad Thing as stated above. If I recall, the horn went off between something like 47% and 97% Nr, so the horn was quiet at idle (you are not going anywhere with the engines at idle) and sounded briefly as you spooled up.
A low-pitch “Dooo-Dooo-Dooo-Dooo” is radar altitude (height above ground). We had one you could manually set, and one that automatically went off at 40 feet.
Was there a voice warning for low ALL pressure?
Nope. 
For that very reason! Engine awl doesn’t even get a light, actually. You have to watch the gay-udge.
If the gay udge was indicating too low, I might start to get ain’ts in my paints about being able to make it into the field without hitting the bob war.
(Anecdote: I worked as a cashier at Kmart after high school. Someone with an Oklahoma/Texas accent asked where he could find the All. I thought he was asking for laundry detergent.)
Wow, so I guessed right?
Going off to bed feeling proud of himself
You did. Back when I was working as a left seater on OH-58s that was the fun sound. It meant the pilot was practicing autorotations. Thankfully I didn’t hear it for real when it becomes the un-fun sound.
In the R22 and Schweizer 300 (TH-55) it’s a ‘warning horn’. Sounds like the stall warning on a fixed-wing: Meeeeeeeeeeeeee!
As you all know, I haven’t been able to afford to fly; so it’s been over a decade since I’ve been in a helicopter. Finally getting my BFR done, I still had the urge to test the low rotor warning during the engine run-up.
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