How far away can you hear a helicopter?

[Mahatatain]

This obviously varies between models of helicopters and the weather conditions but does anyone have any examples or suggestions or thoughts?

For example does anyone know how far away a transport helicopter like the Mi-8 Hip can be heard from on a clear night?

Any rough ideas would be of help.

Thanks,

Mahatatain.

[Legbreaker]

A LOOOONG way if it's as quiet as it should be on a night time battlefield! (i.e. noise discipline is being enforced)
Only takes a poke of the head out the front door when there's a heli in the area to get an idea of distance and time.

[Mahatatain]

Originally Posted by Legbreaker

A LOOOONG way if it's as quiet as it should be on a night time battlefield! (i.e. noise discipline is being enforced)
Only takes a poke of the head out the front door when there's a heli in the area to get an idea of distance and time.

Thanks for this.

Do you have any esimates for what a long way is though? 10 miles? More? Less?

I have no idea (apart from it being a long way.....)

[HorseSoldier]

My last deployment our FOB was visited frequently by ANA and UN Mi-8/17s (plus assorted other flavors of US, ISAF, and contractor helicopters). There was masking terrain about 6-8 km out from the flightline side of the FOB and once incoming aircraft cleared that ridge line you could hear them the rest of the way in, unless there was background noise drowning it out. That's the only time I can think of off the top of my head where I dealt with hearing aircraft coming where I had a known distance terrain feature to index on.

Not sure on a solid figure for how far out you could pick them up if those ridge lines weren't in the equation. Besides masking aircraft on the far side, I'm not sure if the ridges didn't help hear aircraft on the near side by reflecting sound waves back towards the FOB.

[raketenjagdpanzer]

When somebody rabbits from the local cty pokey, or when there's a life flight to the hospital up the road from us, the helo noise will wake the dead: and we're surrounded on all sides by trees and traffic noise.

[stg58fal]

I think there are too many variables to give you a hard number. Helo altitude, model, weather, terrain, background noise, etc. I'd say for game purposes you can hear it as far out as you, the GM, need it to be heard. Or with failures on Perception, Awarwness, whatever, rolls, it can get as close as one ot two hundred meters before someone realizes the bird is there.

[Legbreaker]

Originally Posted by Mahatatain

Do you have any estimates for what a long way is though? 10 miles? More? Less?

What they all said.

In most military situations, it's extremely unlikely a helicopter is going to be able to sneak up on a unit on the ground, unless that unit is making a lot of noise (tank engines, weapons fire, etc). As a general rule of thumb (and to be stretched and altered as the GM sees fit) a 30 second warning would seem appropriate, possibly more if the helicopter is slow moving/large/flat, open terrain, etc, etc, etc

And that's just using the Mark I Ear & Eyeball - throw in some tech (radar, acoustic sensors, etc) and it can get "interesting".

[Trooper]

Finnish air force still has an observer corps (like UK:s former Royal Observer Corps). Unlike cold war ROC their main war time duty is visual detection, identification, tracking and reporting of aircraft over Finland. (Yes FAF + Army AA has more than 100 radars, but observing teams can spot cruise missiles and other low flying targets. And there is also serious doubts how long radar network is going survive (SEAD + ARM:s.))

Anyway FAF tried to replace some observing companies near eastern border with automated acoustic detection systems in 90s. It was great idea- use modern microphones to pick up sounds and use computers to calculate bearing , distance and filter out unwanted background noises. Army paid part of project- they would use passive microphone arrays as warning device against low flying helicopters like Mi-24.

Fine idea, but it didn´t worked out. Acoustic detection of aircraft is unreliable. Computers and modern microphones couldn’t change complex systems of local air flows and winds.

Sometimes you can only hear the plane (jet plane flying over clouds). Sometimes you can`t hear noisy Mi-8 flying less than a mile away.
You should never trust that you can hear choppers or planes. Yes you can hear subsonic jets, but if you detect them by hearing its usually too late. Modern prop planes are quite silent. In motorized or mechanized units there are usually so much background noises, that it makes hard to hear even choppers.

In normal conditions you can spot fighter plane in 3-4 km distance. Aircraft observing is very tedious work, even a single observing post needs a 6-7 men crew.

(Yes serving in Anti-Aircraft Regiment isn´t the most glamorous job in armed forces…)

[Legbreaker]

Of course in a T2K type scenario where there are few to no vehicles moving about (even if they are available, fuel might restrict them to just an hour or so of operation a day), the sound of a helicopter will attract a lot of attention. It's something so out of the ordinary that even the slightest indication of their presence will cause people to sit up and take notice, even if just on a subconscious level.

All in all though, it comes back to the individual situation. There's just too many factors to account for.