Wake Turbulence Separation and helicopters

[[email protected]]

Sorry for intruding onto an ATC forum but I have a question regarding the application of wake turbulence separation between FW and RW.

The reasons for separating FW from FW are obvious since a few knots of differential airflow between one wing and another due to turbulence can be very dangerous leading to stall and loss of control.

On a helicopter, large differences of airspeed exist between one side of the rotor disc and the other all the time, with the rare exception of a still air hover in flat calm conditions, and that difference can be very large, even in the hover on a windy day.

The differences in airflow don't lead to stall or loss of control in a helicopter - even something as light as an R 22 can operate in gusty, bumpy conditions and winds varying far more than in the wake of a 747.

So my question is - why apply the same separation criteria, whether it be distance based or time based, between a FW and a RW?

A second question is, what alleviations are allowed to disregard wake turbulence separation when wind conditions - ie strong crosswind away from other traffic - mean the wake cannot possibly persist and be affecting other traffic?

[jmmoric]

To my knowledge, the primary problem is flipping in case you enter the rotary wake generated around the wing tips of the aircraft ahead.

Other types of turbulence, the one generated from stuff hanging from the aircraft, from preceeding is relatively easy to deal with, as it would in most cases require a slight increase in speed to overcome, it's just as you say, gusty and bumpy.

But entering the rotary wake, which can be very forcefull, is also a problem when flying a helicopter.

[[email protected]]

Even in severe turbulence ie much much worse than encountered near a large jet in take off or approach configuration, a helicopter remains perfectly controllable and has no desire to flip upside down. It might wobble a bit but it doesn't stall or spin.

After nearly 40 years of helicopter flying both mil and civil, mixing it with the big stuff when required, I have never been affected by wake turbulence from a FW.

A 3 min separation from a heavy jet is just massive overkill.

[ShyTorque]

I began flying military helicopters for a living in the first week of 1979 and am still doing so in civilian street. I also hold a CPLA and used to teach close formation in helicopters (no closer than 1 rotor span) and on fixed wing.

In London ATC are now reportedly getting concerned about having to physically separate helicopters from helicopters at the heliport, based on wake turbulence. It's total overkill imho, but as a mere pilot I probably know almost nothing about flying.

"Recommended spacing is..." is still the best way to deal with it. Let the pilot take responsibility and make the decisions.

[jmmoric]

A 3 min separation from a heavy jet is just massive overkill.

Like I said, I really don't know.... I fly Piper 28 and control traffic, have no experience in helicopters. But my understanding of the wing tip vortices is that it creates a circular motion of the air, and that circular motion can be strong enough to flip stuff... though I don't see why it shouldn't affect helicopters?

Some countries allow for the pilots to take the responsibility for wake turbulence separation, if you are able to do so, you'll get "essential traffic information" about the wake turbulence, and a take-off clearance, and off you can go.

[ShyTorque]

jmmoric,

The wings on helicopters aren't fixed, for one thing. If the rotors fly up or down, the fuselage doesn't necessarily follow.

The only time in 42 years of helicopter flying that I had a problem, albeit very short lived, was about thirty years ago when we were on very short finals to pick up an underslung load alongside a runway and a Chinook flew directly over us, probably less than 100 feet above, when he misunderstood his landing clearance and approached the wrong landing spot.

[jmmoric]

jmmoric,

The wings on helicopters aren't fixed, for one thing. If the rotors fly up or down, the fuselage doesn't necessarily follow.

Makes sense.

Unfortunately I can't change rules.

[bobward]

One of our low hour students got too close to a helicopter on finals a few months ago. The turbulence from the helicopter flipped the aircraft, about 30 degrees I'm told. Not the sort of thing you'd want if a student, or even an experienced plank winger. It seems that the student was allowed to get a bit too close to the heli, with the above result.

[ShyTorque]

BW, Yes, that situation is well documented and not at all surprising; the downwash from a helicopter can be surprisingly intense. There was a tragic, fatal accident at St. Mawgan/Newquay some years ago where the pilot of a light aircraft went around from an approach and its pilot subsequently lost control after allegedly being affected by downwash from a RAF Seaking operating alongside the runway. The RAF pilot was an acquaintance of mine and the burden of blame placed on him was, in the circumstances, unfair. I don't think he ever got over it and he passed away in sad circumstances at a rather young age.

Crab was referring to the other situation, i.e. how turbulence from a fixed wing tends to affect a following helicopter to a far lesser extent.

[AlanM]

Yeah, I’m with Crab and ST on this 1 as an ATCO it seems pointless - not least when you see helicopters on SAR tasking in Gale Force conditions and hocering happily (or operating the North Sea Rigs etc)

Rules are rules......... and they have added tiltrotors into that as well now. (Obvs we get a lot of them!)

Bizarelly, when we operated the Helipad at the Farnborough Airshow we had heavy depatures off runway 24 with the 25 FATO converging on the runway with no worries (and aircraft parking/manouevring inbetween)


https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/854x888/1223f79d_f7e6_4b0c_addf_ccc8715447bb_3a5cfe176ac214533dd27b3 fee9257a5db506f67.jpeg

[Ant T]

"Recommended spacing is..." is still the best way to deal with it. Let the pilot take responsibility and make the decisions.

At Aberdeen, still one of the busiest airports for helicopter movements even with the downturn, that is what happens, and in my experience, it is acknowledged and that is the end of it!
Is it ever mandatory?

[Gonzo]

1) Because we’re told to (agree it’s overkill for helicopters)

2) Not in the civvy world, although there are a few concepts out there in the R and D phase to reduce or completely negate wake turbulence separation when there’s a crosswind greater than 7kts+, but these concepts are based on fixed wing pairs departing from a runway on SIDs.

[[email protected]]

I get the impression that there is something of a grey area regarding this requirement to apply FW-FW separation rules to FW-RW - where are the rules that mandate this application of separation?

Shy - didn't realise you knew PB - that crew at St Mawgan were all my colleagues at Chivenor at the time and I still work with the Rad Op. The downwash allegation was nonsense and, when showed to be so, the family (seeking compensation) changed their argument to propose distraction caused by the proximity of the Sea King.

[Wycombe]

I recall an awful accident at Oxford in the early 90's when a light aircraft (PA28, I think) was flipped on short finals by the downwash from a large helicopter (S61, I think, which was operating shuttles from there as it was British Grand Prix weekend).

I remember hearing about the accident on the radio news, and a few days later that the instructor killed had been an instructor of mine at Brize Norton Flying Club.

[Capn Bug Smasher]

I recall an awful accident at Oxford in the early 90's when a light aircraft (PA28, I think) was flipped on short finals by the downwash from a large helicopter (S61, I think, which was operating shuttles from there as it was British Grand Prix weekend).

Here (https://www.google.com/url?sa=t&source=web&rct=j&url=https://assets.publishing.service.gov.uk/media/5422eb4de5274a1314000079/Piper_PA-28-140_Cherokee__G-BRWO_07-10.pdf&ved=2ahUKEwj06dSajIXwAhWJaM0KHfkJBVQQFjAAegQIBBAC&usg=AOvVaw2_sYSB3C0Yep_YMt8XzKtH&cshid=1618656225637) is a link to an AAIB report for a PA-28 crash on short final believed caused by wake turbulence from an S-76 in 2009, for interested parties.

I have only a university aerospace engineering degree and yes, rotor blades flap - as I recall, to mitigate dissymmetry of lift as the aircraft moves - but is there a limit to the hinge angle, which, if exceeded, could cause the helicopter to roll anyway?

Could a wake vortex act on the rotor and fuselage independently? I suppose there must be a distance at which the vortex diameter is about the same as the length from the rotor head to the fuselage, so the top of the vortex could grab the rotor and the bottom the fuselage? Although I suspect this distance would be so close to the landing aeroplane that the helicopter pilot would have bigger problems anyway.

Edit: after a quick research, no, don't think so. I just read a paper where the vortex core was measured and it's only about a foot across for a 757 / a bit more for an MD-11. Presumably the velocity outside of the core rapidly decays to a point that wouldn't bother a helicopter

[terrain safe]

Look here MATS Part 1 (https://publicapps.caa.co.uk/modalapplication.aspx?catid=1&pagetype=65&appid=11&mode=detail&id=6657) and see from page 103 onwards.

[Pugilistic Animus]

I'm definitely not a helicopter pilot, I do find them interesting. I'm quite surprised that wake turbulence, that would easily down smaller jets and airplanes, isn't a worry in the RW world!

[whowhenwhy]

I was going to highlight the MATS Part 1 text so thanks for that.

​​​​​​The problem is that whilst we know that helicopters can cause wake vortices, little or no research has been done on how helicopters are affected by these vortices and the turbulence they create and how well they can resist those effects. As such, the wake turbulence separations are applied equally to both fixed and rotary wing.

[[email protected]]

The problem is that whilst we know that helicopters can cause wake vortices, little or no research has been done on how helicopters are affected by these vortices and the turbulence they create and how well they can resist those effects. As such, the wake turbulence separations are applied equally to both fixed and rotary wing. And that seems to be the problem - no-one wants to listen to helicopter pilots who know the wake turbulence from a FW just isn't a problem - downwash from another helicopter is a different matter and we all know about how to avoid that.

Even the MATS pages (thanks for the links) specify in the opening paragraphs that wake turbulence vortices are most hazardous to aircraft with a small wingspan during the take-off, initial climb, final approach and landing phases of flight - nothing there about affecting helicopters at all.

It just seems a very lazy option to apply FW limitations to RW operations.

If you are trying to complete an instructional sortie within an allotted time (because the customer is paying) and you are constantly held and delayed for wake turbulence, it is a frustrating and inefficient restriction of airspace, but it seems the controllers hands are tied because the authorities can't be bothered to acknowledge that helicopters are different.

I am led to believe that the FAA don't apply wake turbulence separation to helicopters on approach behind a FW.

[ShyTorque]

It’s by no means unusual for ATC to hold our heli on the runway prior to departure, in CAVOK conditions, for a preceding fixed wing departure in a strong “away” crosswind; even though it’s perfectly obvious from the adjacent windsock that wake turbulence from that aircraft was never really over the runway and well within three minutes it wouldn’t even be over the airfield.

Thing is, when hover taxiing (or arguably, partly so, even when ground taxiing), a helicopter has already taken off to get to the runway departure point. To subsequently hold it for wake turbulence reasons is absurd.

[whowhenwhy]

Crab, fully acknowledge the issue that you're describing but the requirements for separation are sourced from ICAO and have been reinforced in recent years by research undertaken by the FAA and EUROCONTROL; albeit that research focused on fixed wing.

The CAA did some work on RW wake turbulence a few years ago (which resulted in a minor amendment to the ICAO materials, I think a note was added) but it focused on RW AS generates of wake, not how they react to wake. I believe that there was a plan to do some (UK) research on this aspect a few years ago but the funding application fell through.

[[email protected]]

whowhenwhy - as I mentioned earlier, every helicopter pilot knows the dangers of another helicopter's rotor wash but despite being of similar strength, especially things like S-92 and Merlin, to a large FW, the vortices do not persist and dissipate in a very short distance from the aircraft.

Even if you do encounter the wake of another helicopter, there is no loss of control or stall, it usually just affects your power required briefly.

There seems to be a dearth of evidence or any research to produce such evidence that would lead you to apply a FW safety protocol to a RW aircraft.

As I mentioned, a 3 min hold is complete overkill for a helicopter to transition after a heavy or super FW has rolled or gone around.

[jmmoric]

I am led to believe that the FAA don't apply wake turbulence separation to helicopters on approach behind a FW.

As I mentioned, a 3 min hold is complete overkill for a helicopter to transition after a heavy or super FW has rolled or gone around.

ICAO states that ATC doesn't have to establish wake turbulence separation between VFR flights and preceding heavier aircraft, we'll provide the caution, then the separation rests with the pilot. Same as with consequetive IFR flights on visual approach maintaining their own separation.

The 3 minutes hold is again a requirement by the rules, and some countries allow for the pilots to accept separation to a preceding heavier departure (but I think it has been removed in DOC4444?)... there is no standard phraseology for it though, and I've heard people saying that if the pilot just says he is "ready", it means he will take the wake turbulence separation himself.... though I personally don't like that interpretation.

[[email protected]]

ATC usually provide the recommended spacing for VFR approaches but mandate the time for departures - haven't ever heard of allowing the pilot to make his own decision on a departure.

[jmmoric]

ATC usually provide the recommended spacing for VFR approaches but mandate the time for departures - haven't ever heard of allowing the pilot to make his own decision on a departure.

You could ask tower if it is an option to take that separation yourself, I don't believe they would ask you first on that one, since you may inadvertently end up putting a pilot in harms way (if they misunderstand the intention)? I don't know about the local rules for wake turbulence separation for VFR where you fly though?

[[email protected]]

I don't understand why there should be 'local rules' - surely any difference from ICAO procedures would have to be sanctioned nationally by the CAA.

Any controller is going to play by the rules since it is their licence on the line - it is the rules that need clarifying on this matter.

We have a very good relationship with our local ATC but the increase in traffic now many covid restrictions have been lifted has brought the situation into sharper focus.

[Dan Dare]

it is the rules that need clarifying on this matter

The rules are quite clear, any controller should know exactly which rules they’re applying and I would expect any pilot to know what wake turbulence actions they should take. Sometime there is a mismatch between what the pilot is happy with and what the controller is permitted to apply. It’s a pity the rules don’t always match the real world risk and that there are so many variation of rules depending which flight rules, airport or state you fly from.

[[email protected]]

The rules are quite clear, any controller should know exactly which rules they’re applying The rules do not stipulate FW separation to be applied to RW - it is just assumed.

[alfaman]

The rules do not stipulate FW separation to be applied to RW - it is just assumed.
That's not my understanding; if an aircraft, FW or RW, is allocated a wake turbulence category, then ATC are obligated to apply it, when required to do so.

[jumpseater]

The rules do not stipulate FW separation to be applied to RW - it is just assumed.

The wake vortex is based on weight categories used by ATC L light/ S small /M lower and upper medium /H heavy. Each type including helos have a category allocated to them. Eg R44=L S92=S a320 =M etc. There is no distinction between rotary or FW in the application of the MatsPt1 tables S1 Ch3 P13 also section 9k a bit further down from this link.
https://publicapps.caa.co.uk/docs/33/CAP493%20Edition%206_1%20Corr%20(April%202015)%20Complete%20 9780117928886.pdf
So a controller shall apply those separations, and they are drummed in under training. Also note separation standards required for parallel runways in said documents. Then ask how does that work with a parallel taxiway and a heli taxiing with skids, eg MD’s Robbo’s,...

A controller has to use those separations, so if you’re at an airfield where rotary has to use the runway for departures and the traffic mix is different weight/wake vortex categories, (and speeds), it can be as frustrating for controllers as it is for the engine drivers...

[Equivocal]

As others have pointed out, the rules (certainly for the UK) are clearly set out in the national procedures manual. It is unfortunate that at aerodromes where RW mix with larger FW operations those procedures do appear to penalise the RW operations, in some cases quite significantly. Sadly, simply being unhappy with that and posting here doesn't change things. But there is a process for changing things, at your home base, at least, if your ATC people support it. Many national procedures can be varied to take account of local conditions and features....if the variations are published in the local Manual of ATS. To get something into the local procedures it will have to be approved by the CAA and I imagine this will require something along the lines of a safety argument showing that the variation is at least as safe as following the 'standard' rules. Whilst this takes some work and effort, if there are benefits to be gained by both RW operators and ATC (and, probably, the airport), maybe it's worth doing. I've no idea how such things are viewed these days, or how practical it is at your base and with your aircraft, but there may be other possibilities which do not require some of the separations to be applied. In the past I recall one airport where RW (although not larger ones) hover/air-taxied from the apron to a designated point in a grass area, from which they transitioned and set course largely as desired. The point was quite specifically not a FATO and was outside the runway strip - no wake turbulence separation is specified and a caution was issued if the aircraft was going to pass behind or through the recent path of another aircraft. Seemed to work OK, I'd go further and say it worked well for everyone, but maybe it's been stopped in the intervening time.

[[email protected]]

Thanks for all the replies - I know what the rules say but the whole argument for wake vortex separation is based on FW encounters with it not RW and especially the dangers to a FW with a small wingspan getting trapped in the vortex and losing control.

It doesn't mention the dangers to RW because they don't know if there are, it is all based on FW research.

Rather than do the studies to see exactly what effects a wake vortex has on a RW, it has just been assumed and read across - the MATS manual specifies FW susceptibility but only mentions helicopters as a generator of wake vortex, not a potential victim.

It seems to me that classifying helicopters in the same way as FW is all about their ability to generate wake, not their susceptibility ie a S92 is far more dangerous to a light FW than an R 22 dues to its weight and therefore downwash strength.

I'm not expecting this situation to change because once you apply a safety measure, no matter how pointless, the amount of work to get it removed in our risk-averse world is unlikely to be completed and fought at every turn.

I have written to the CAA but I won't be holding my breath - I will be advised to read MATS and pointed out that the rules are the rules.

Meanwhile I'll just lose time and revenue waiting for an almost non-existent threat to my aircraft to dissipate, even in conditions where it is physically impossible to affect me.

[Equivocal]

Glad you got that off your chest.

[[email protected]]

Well I rather hoped that some educated ATC er would point me in the direction of the evidence used to establish these rules with helicopters rather than just pointing out what MATS says.

[callum91]

Well I rather hoped that some educated ATC er would point me in the direction of the evidence used to establish these rules with helicopters rather than just pointing out what MATS says.

If it is a problem at the airport you’re based at maybe try to approach the airport authority and ATC unit. They they may be able to improve their procedures for example providing dedicated areas for helicopters to operate in which are deemed separated for wake turbulence purposes (see https://www.pprune.org/atc-issues/636777-wake-turbulence-seperation-helipad-less-than-760m-runway.html (https://www.pprune.org/atc-issues/636777-wake-turbulence-seperation-helipad-less-than-760m-runway.html?highlight=Gonzo)) and cross helicopters where they won’t be crossing the flight path of the fixed wing.

[[email protected]]

Done all that Callum, the geography doesn't lend itself to any areas for circuit arrivals and departures away from FW. We do have manoeuvring areas for ground cushion work but that is it.

[alfaman]

Well I rather hoped that some educated ATC er would point me in the direction of the evidence used to establish these rules with helicopters rather than just pointing out what MATS says.
Your original post asked two questions:
1) why apply the same separation criteria, whether it be distance based or time based, between a FW and a RW?

2) what alleviations are allowed to disregard wake turbulence separation when wind conditions - ie strong crosswind away from other traffic - mean the wake cannot possibly persist and be affecting other traffic?

The answers are above, really: 1) because the rules require it; 2) none, because the rules require it.

You're not going to get a response here that helps any more than that, I'm afraid, because it's not within the gift of those posting here to change it. It doesn't matter whether they want to, or not, they don't have the authority. I'd be staggered if any airfield operator was prepared to sign off a safety case that changes any of that, without clear documented evidence from a higher authority, so your best bet would be lobbying via the CAA, & the BHA perhaps.

[[email protected]]

alfaman - yes, the answers were what I was expecting, I just wanted to know if anyone had any inside knowledge of studies or trials that actually support the rules as they stand.

[terrain safe]

alfaman - yes, the answers were what I was expecting, I just wanted to know if anyone had any inside knowledge of studies or trials that actually support the rules as they stand.

The question could also be; Do you know of any studies or trials which prove the rules are not required?

[jumpseater]

alfaman - yes, the answers were what I was expecting, I just wanted to know if anyone had any inside knowledge of studies or trials that actually support the rules as they stand.

I can’t recall hearing of any tests/trials that determined departure separation standards for rotary vs FW. My guess is Rotary and FW were categorised purely by weight in establishing the rules.
If a change were to be proposed, there may be a baseline platform data of military ops, as, as I understand it UK mil aren’t ‘restricted’ in the same manner. How to access that data, if it exists and if it would be considered meaningful are way above my pay grade. But if you wanted a place to start with examples of safe operations, that might be a path to follow.

[[email protected]]

Terrain safe - just 40 years of helicopter experience, mostly military, and a fair bit in very turbulent conditions in mountains. I have also operated in and around FW - FJ and ME - and never encountered anything more than a burble from wake turbulence - even in and out of LHR.

Jumpseater - good point, I'll follow that up.

[ShyTorque]

The question could also be; Do you know of any studies or trials which prove the rules are not required?

Surely, if that was the case there would be no need to post this thread.

But some operators routinely fly helicopters in very close formation with much larger fixed wing, as here for example:

https://youtu.be/ibM7DA_b6OM

[terrain safe]

Terrain safe - just 40 years of helicopter experience, mostly military, and a fair bit in very turbulent conditions in mountains. I have also operated in and around FW - FJ and ME - and never encountered anything more than a burble from wake turbulence - even in and out of LHR..

I've been a controller for over 30 years and after applying the rules never had a problem either. So therefore the rules work well and keep everyone safe.

ShyTorque But these 2 aircraft are in the same wake turbulence category and so require no wake turbulence spacing between them. I don't think that this is done for smaller helicopters (but accept that it's mission dependant rather than size).

[ShyTorque]

I've been a controller for over 30 years and after applying the rules never had a problem either. So therefore the rules work well and keep everyone safe.

ShyTorque But these 2 aircraft are in the same wake turbulence category and so require no wake turbulence spacing between them. I don't think that this is done for smaller helicopters (but accept that it's mission dependant rather than size).

I’m surprised to hear they are in the same category. A Blackhawk weighs 10 tons and a C-130 weighs up to seven times as much. Yet I have heard that an AW139 (approx. 6.5 tonnes) will now require wake turbulence separation from helicopters such as a 109, max weight 3.175 tonnes.

[jmmoric]

I’m surprised to hear they are in the same category. A Blackhawk weighs 10 tons and a C-130 weighs up to seven times as much. Yet I have heard that an AW139 (approx. 6.5 tonnes) will now require wake turbulence separation from helicopters such as a 109, max weight 3.175 tonnes.

Light: 7000 kg or less.
Medium: between 7000 kg and 136000 kg.
Heavy: 136000 kg and more.
A380 has it's own category.

In some countries the B757 is weighed as a heavy when flying in front of another aircraft, and meduim flying behind.

Do keep in mind that these are MTOW. And there has been some talk about changing/adding more categories.

By local rules I meant the rules the local CAA has decided on, besides what ICAO say.

[Dan Dare]

So in the U.K. alone depending where you fly from you could receive “caution wake turbulence, the recommended spacing is...” or separation applied (or not) variably based on CAA MATS Part 1, USAF, RAF, individual MATS Part 2 eg RECAT/CREDOS/TBS rules each subtly different from ICAO never minding local rules on what constitutes an intersection, relevant proximate runway or taxying near a runway. No wonder the question comes up.

Are there any examples of helicopter incidents due to fixed wing wake turbulence? I don’t remember any.

[jmmoric]

Are there any examples of helicopter incidents due to fixed wing wake turbulence? I don’t remember any.

Me neither..... and I've seen VFR helicopters joining in very tightly behind heavies counting on us to use "reduced runway separation".

The fun part is, we can only use "reduced runway separation" when the braking action isn't affected....... which makes a lot of sense when no. 2 is a helicopter....

[ShyTorque]

Even more so when a helicopter is mandated to use a runway that it doesn't need and then given mandatory separation...

The traditional "Standard Helicopter Join" of approaching at ninety degrees to the centre of the active runway seems to have been totally forgotten.

[jmmoric]

....

To be fair, the design of the runway, including obstacle free zones etc. are designed for all aircraft, those doesn't really change just because you're in a helicopter. Though we agree a helicopter can land on a dime, and won't need a runway.... the runway is still the part of the airport "designed for the landing and departure of aircraft". Unless helipads have been established.

But yes, we can ask for a lot from you guys, and it's appreciated.

[ShyTorque]

jmmoric,

We realise it's not the fault of the ATCO on the day. But with DOC about £30 a minute for a light twin heli, we're very much aware of how things could often be carried out in a more expeditious way without compromising safety.
An ATCO who has never seen how helicopters operate once away from an airfield, would really not understand how small an area of real estate a modern helicopter needs to take off and land, even whilst complying with with Class 1 performance standards. For example, an AW109 only needs an operating area of approx. 50 feet x 50 feet.

[[email protected]]

The fun part is, we can only use "reduced runway separation" when the braking action isn't affected....... which makes a lot of sense when no. 2 is a helicopter.... Another indication that FW rules have been applied to helicopters without any real thought.

[whowhenwhy]

Crab, the research clearly needs to be done to focus on RW, and, based on what you and ShyTorque have said, it sounds like there is an opportunity for a more flexible approach to be applied to RW.

We need someone to fund the research though. Neither ICAO nor the CAA have the money. That means either a university with a research grant (and many of their sources of cash dried up when we left the EU) or business; BHA?

[ShyTorque]

Another indication that FW rules have been applied to helicopters without any real thought.

Maybe some days the air is more slippery ;)

[[email protected]]

By way of closing the loop, so to speak, I have had a great response from the CAA following an email question I sent.

There is little empirical evidence that FW wake turbulence affects helicopters in any major way and one study, done by the well-renowned Gareth Padfield in the late 90s, seems to be the basis for applying FW separation criteria to RW.

The problem is that it is theoretical, using vortex simulation algorithms and a Lynx Mk 3 s handling characteristics - a good place to start but it assumes a lot of 'worst-case' scenarios (no vortex dissipation and no pilot intervention for several seconds for instance) and uses a Helicopter handling qualities standard (ADS33) to predict the disturbances in pitch, roll and yaw caused by interaction with the vortices.

Again, good place to start but there seems to have been no attempt to follow it up with real-world testing so anyone reading the study - presented at the 25th European Helicopter Forum - would think that any encounter with any wake vortex would be very hazardous, not something backed up by any empirical data and certainly not confirmed by any anecdotal evidence from real encounters.

Once such a safety protocol is put in place, it becomes near impossible to roll it back or water it down without a huge amount of effort and evidence and the RW industry isn't interested in fighting it's corner here it seems.

It was a useful exercise asking the question and I really wasn't expecting anything to change but I scratched the itch.

Many thanks to Rob at the CAA and to all who have contributed.