Rotor downwash

[tmmorris]

Flying as I do from an airfield with a lot of big helicopters :-) made me wonder if there is any official (or at least useful) document on rotor downwash? There's the AIC on wake turbulence (http://www.nats-uk.ead-it.com/aip/current/aic/EG_Circ_2009_P_064_en.pdf) but there doesn't seem to be an equivalent one on the effects of rotor downwash on light aircraft.

ATC often give 'caution rotor downwash' (e.g. taking off yesterday behind a big helo which had been hovering mid-runway, and landing behind one ditto).

I bounced on that first landing yesterday and had to go around but I don't think that was the rotor downwash :hmm:

Tim

[Torque Tonight]

Rotor downwash is greater than that from an aeroplane of equal weight and so light aircraft must treat all helicopters with a great deal of caution. The vortices will move and behave in a similar way, just with greater strength. As with an aeroplane, the vortices are stronger at lower airspeeds and therefore the worst case likely to be encountered is from a helicopter in the hover. Even a helo with rotors running on the gound may still be generating some lift.

In my previous life, flying helicopters even larger than those that I believe TMM encounters, we were acutely aware of the damage we could do to light aircraft. Close to the aircraft, the wind generated by the downwash was in the region of 100kts - more than double the typical stall speed of a light aircraft. There is a video knocking about of a light aircraft taxying too close to a hovering helicopter and ending up on its back. Give 'em a wide berth.

[24Carrot]

If it's any consolation, little helicopters get worried by big helicopters too.:eek:

[AdamFrisch]

Rotor downwash on a gyro is very dangerous as it relies completely on the rotor being in an autorotative state. If the airflow gets reversed, the rotor stops flying. At least one crash is accounted to this. Helicopters have this same problem, but most of them can power out of it (most of the time).

I have not encountered rotor vortices when flying FW, but I can imagine they could be rather hairy given the right circumstances.

[tmmorris]

Thanks - so yesterday's scenario (Merlin hovering on a virtually windless morning) is about as bad as it gets...

I have so far treated it as one would wake vortex i.e. take off from a point before the helo's takeoff position and maintain a flight path above where it has just been. Does anyone know how far and how fast the vortices spread outwards? (I assume they do, like wake vortices?)

Tim

[24Carrot]

Thanks - so yesterday's scenario (Merlin hovering on a virtually windless morning) is about as bad as it gets...Bad for two reasons:

1) the turbulence stays put

2) no translational lift, so that Merlin was at max power for the weight (for hover IGE)

[aluminium persuader]

Grab a driver of one of said beasties & get him to explain things to you. Failing that, pop up to ATC & have a chat with the duty pilot. There are a few more things to consider eg in/out of ground cusion, proximity to buildings etc which can be interesting.

[Pilot DAR]

While agreeing with what has been said so far, I'll add, that if you're stopped in a place you're supposed to be, the rotor downwash responsibility becomes that of the helicopter pilot.

[RatherBeFlying]

The Citabria I used to fly was based at a helo training facility. After an interesting landing when a small trainer was hovering in the vicinity of the threshold, I decided not to do that again:\

[ShyTorque]

tm,

Having instructed on both military 7 tonne helicopters and light aircraft, including close formation, I can confirm that the rotorwash definitely goes downwards; so aim to stay above it and you will be OK. If you ever see military helicopters in close formation, you will see the following aircraft "stepped up" from the ones in front, for this reason.

On the other hand, RAF fixed wing formations usually "step down" from the ones in front.

To put this in perspective, our regulations (RAF) allowed a minimum of one rotorspan between similar helicopters in formation.

Do beware the danger from downwash on your flying controls on the ground - hang onto them tight, if operating near taxying helicopters, even though your aircraft isn't flying! If you don't, the rotor downwash may blow your control surfaces to full deflection and this will possibly cause untold damage to your aircraft.

[Sir Niall Dementia]

Rotor vortices are known to have caused at least two fatal crashes to FW in the UK.

I fly medium sized rotary for part of my day job and I treat the situation of mixing with light FW very carefully.

I used to fly the Sikorsky S61 round the North Sea and was once told by a Viscount captain that hitting our wake on finals to Sumburgh was like being stamped on.

SND

[cjhants]

there was a thread - not sure here or elsewhere- about 5 years ago on a fatal accident in the west country, cornwall i think.

at about that time, as a newly qualified PPL i was blown sideways by a metre or so in the flare, i believe by the wash from a large helicoptor at a taxiway holding point, which was close to my aiming point. no damage done, but valuable lesson - aim further down - land long - keep well away from those blades.

[astir 8]

There's a helicopter vs. PA28 (G-BRWO) accident report in the latest (July) AAIB bulletins worth reading.

Also reference to a 1992 accident reported as AAR 1/93

[Senior Pilot]

You will get much more accurate information if you ask this on Rotorheads. (http://www.pprune.org/rotorheads-23/) There is a degree of misinformation in a couple of posts, and you'll get a lot of proper responses from us rotary types ;)

[tmmorris]

Thanks for that suggestion - wilco.

Tim

[Heliport]

cjhants there was a thread - not sure here or elsewhere- about 5 years ago on a fatal accident in the west country, cornwall i think.

http://www.pprune.org/rotorheads/199266-mod-pays-5-million-plane-crash-victims-family.html
http://www.pprune.org/military-aircrew/198994-mod-pays-5-million-plane-crash-victims-family-update.html
http://www.pprune.org/rotorheads/212675-inquest-rotorblade-downwash-did-not-cause-plane-crash.html

Cessna 182S, G-WHDP
23 June 2001
RAF St Mawgan, Cornwall

AAIB Conclusion:
It was concluded that the Cessna crashed following a loss of control during an attempt to carry out a 'go-around'. The activity of the Sea King and the loss of control by the pilot of the Cessna were two distinct, and for the most part, unrelated events occurring at the same time. The plot constructed of the relative positions of each aircraft considered in conjunction with the rotor downwash trial established conclusively that the rotor downwash from the Sea King did not play any part in the Cessna leaving the side of the runway.

The possibility that the pilot of the Cessna was distracted by the actions of the Sea King could not be discounted. If that was the case, the option to stop on the runway by closing the throttle and applying the wheel brakes was available. The higher than normal speed maintained by the Cessna after landing was more likely an attempt to expedite clearing the runway than trying to take avoiding action. It is possible that expeditious runway clearance was either not to delay the departure of the Sea King or to reduce the time taken to travel to park at the Civil Air Terminal at the far end of the runway. The loss of directional control on the runway was attributed to the high ground speed and the attempt to carry out a 'go-around'. The go around was probably attempted to resolve the difficulties, experienced by the Cessna pilot, in maintaining directional control.

The outflow of air from the Sea King's downwash possibly affected the Cessna, when it became airborne and crossed vehicle access M. The turbulence created by the downwash could have added to the Cessna pilot's control difficulties. It is also possible that the low speed and increasing high angle of attack of the Cessna as it rotated into the air may have caused the aircraft to stall and descend quickly after becoming airborne.

"The outflow of air from the Sea King's downwash possibly affected the Cessna, when it became airborne and crossed vehicle access M"
ie After the pilot had lost control, veered off the runway (taking out a runway light in the process), was going across the grass heading towards a large hangar and briefly became airborne.

Why he didn't chop the throttle/use his brakes either while on the very long and very wide runway, or while travelling across the grass, are questions only the pilot would be able to answer. Sadly, he didn't recover from injuries sustained in the subsequent fire.


.

[Whopity]

Some years ago a colleague was killed when his PA28 did a touch and go after a large helicopter had hover taxied across the runway. It happened on RW27 at Oxford on the day of the Silverstone Grand Prix. He was an experience RAF Bulldog QFI and was about to take up the role of Head of GA at Oxford.Details appear in the AAIB report AAR 1/93 dated 10Mar93 (http://www.aaib.gov.uk/cms_resources.cfm?file=/1-1993%20G-BPJT.pdf).

[SNS3Guppy]

I had an encounter while landing in a Cessna 337 a year or two ago, with the rotorwash from a little Robinson R22. It was night, and the Robinson was about fifteen feet in the air, nearly invisible, and taxiing clear of the runway. I passed behind it, landing, and nearly thought I was sideways. Correction required power and nearly full control input until clear.

I've ended up flying close, under helicopters at night, while in a fixed wing light airplane, when the helicopter had no lights on. The results were dramatic, roughly akin to flying through a large dust devil, crossed with a microburst.

[Captain Smithy]

Interesting one in this month's AAIB bulletin :ouch: Never something I've had any experience with, certainly has given me food for thought, something to bear in mind for the future.

[L'aviateur]

Was landing today in a PA-28, whilst a Kamov KA-32 (I think) with an underslung load was in the hover, approximately 50m lateral to the runway.
I seriously underestimated how much effect this helicopter would have on me, and had I been any closer would have struggled to recover the situation.
I now appreciate the rotor wash effect from helicopters much much much more.

http://www.aaib.gov.uk/cms_resources.cfm?file=/1-1993%20G-BPJT.pdf

iii) The dangers caused by rotor downwash generated by large helicopters when hovering close to an active runway have not been sufficiently researched. Although helicopter downwash and wake vortez is recognised as being hazardous it may be that such conditions associated with hovering helicopters are not widely appreciated.

Something of an understatement, I really wish i'd read this AAIB report before.