"Flicker Vertigo"

[DualDriver]

Hi Guys

I'm trying to get some info regarding this phenomenon. Is there anyone out there that can pass on some knowledge about this.

Thanx

DD:ok:

[joyrider]

worst on a sunny day when you've got a hangover:\

[Mr Toad]

Some of our pax in the North Sea used to experience this on Chinook flights when sitting next to the window on the sunny side. It is related to epilepsy I believe; some people are more prone than others. The frequency of the flicker ing sun through the large, slow-moving rotors seems to upset the brain's natural frequencies - alpha waves I believe; the seizure that follows is extremely alarming to both the victim and to those sitting nearby. You could say that this might happen to anybody who sat in a Chinook for five hours but that would be unkind. I never saw it on the 61 or the 76; nor on any Bell I flew. Don't know if it happens on military Chinooks.

The pilot can remove the source of the problem simply by moving the pax to another seat or by changing course to take him out of the sun. Usually these guys don't get to fly again once their companies hear of an occurrence; sad for them as it affects their job.

For aircrew the matter would be serious though; I remember the Dutch RLD tests pilots for sensitivity to flicker effect as part of the initial aircrew medical. I don't recall the CAA doing this.

Any flying AME out there? Roger Green or anybody?

[212man]

Sadly, Roger Green died several years ago.

I thought part of the reason for the initial EEG was to detect any proneness to this effect.

Global warming must have changed tyhings since the Chinook days on the N. Sea; sun light isn't too much of a problem these days!

[Ascend Charlie]

Haven't had vertigo or epilepsy from it, but have felt very fatigued when enduring rotor flicker. Turning my cap to cut out seeing the rotor helps a bit (looks a bit creepy, though) and still can't avoid the flicker reflecting off kneeboards or instrument panels.

Never seen it affect anybody in the aircraft, either two-blade or four.

[212man]

"So divide your Rrpm by 60 and see how susceptible you are."

Wouldn't that only apply to a single bladed rotor? Surely you need to multiply that by the number of blades, as that's how often the sun is going to be blocked, per second?

Where does your figure of 17 for a Huey come from? using your calculation you imply the RPM is 1020.

Interestingly, though, I find the 155 gives me some uncomfortable effects sometimes if looking through the disc or at the nav log and the sun is directly above. If your flicker range is correct, then this ties in as the 5 blades and 342 Nr gives a frequency of 28.5 Hz.

[DualDriver]

Thanks for all that

From what I've read before, it usually occurs when flyin into a setting sun with the sun about 30 degrees above the horizon. How accurate is that?

More inputs, please

DD

[jayteeto]

One of the blokes on my shawbury helicopter course was chopped for this after losing control on instruments. He said the blade flicker with the bright sun made him lose concentration. Docs said flicker vertigo=epilepsy and he was out. Took ages to persuade the CAA he wasn't epileptic. He is an airline pilot now!!

[NickLappos]

Here is the straight word on flicker vertigo, a paper from the Flight Safety Foundation. You have to sign up, at no cost, to download it:


http://www.flightsafety.org/members/serveme.cfm?path=hf/hf_mar-apr04.pdf

How about flicker vertigo from cartoons!

http://news.bbc.co.uk/1/hi/health/651224.stm

[Vfrpilotpb]

It is connected with Brain Frequency, it can affect drivers at a certain speed with fencing or tree trunks which just at that moment of travel coincide with the correct roadspeed to give the right speed of flicker.

Has caused some fatal impacts in the past, but only a few inquests have related to this Syndrome as the culprit!

* Night clubs and flashing strobes can also trigger this situation
Vfr

[KENNYR]

It happened to an Army Lynx pilot in the early 80's at Aldergrove. He was on dispersal with engine running when he went into epileptic fit mode. Luckily a technician was on board and shut the aircraft down.

[SASless]

During WWII, flashing lights were used as a tactic to combat night bombers as I recall....also quite familiar with the problem from my Chinook days in the US Army and again with the S-58T....nothing to do with epilsepsy as I recall. The worst I ever got was a feeling of discomfort....or uneasiness....the Chinook was worse when the flicker reflected off the Attitude Indicator...with a sun angle from slightly behind the aircraft...usually shortly after lunch.

[[email protected]]

I believe certain roads in rural France have trees planted at exactly the right interval to induce this problem in susceptible individuals driving at normal road speeds. I don't think they did it deliberately but you never know with the French!!
The initial military medical used to have a test for flicker epilepsy - when wired up to the EEG, a doc would jam a strobe light into your face and see if you went wibble!

[Figure Of Merit]

There are 2 separate problems that can both be caused by bright sunlight shining through the disc. Quite a few of the posts confuse the two.

The first problem is induction of an epileptic electrical activity in the brain by a bright flashing light. Some individuals (the CAA quotes 0.5% of applicants) are susceptible. The usual frequency range that causes problems is 1 to 30 Hz and in the UK an EEG with photic stimulation in this range forms part of the initial Class 1 medical exam. You're either susceptible or you're not. If you do get epileptiform waveforms, you have a greater than 1% per annum chance of having a seizure and are therefore not considered medically fit.

The second problem is a separate one and is nothing but an optical illusion that can be very disorientating. As the sun shines through the disc, the eye sees a large shadow sweeping across the visual field rapidly and repetitively. The brain "believing" that the visual field is moving across the retina interprets this as meaning that the body is moving in the visual environment (rather than the visual environment is moving across a steady body). The conflict between this and other cues to spatial orientation causes a disorientation that is true flicker vertigo.

You can demonstrate the effect by getting someone to stand infront of a disc that is painted with a pattern. If they're close enough to the disc such that the pattern fills most of their visual field you can get them to fall over just by rotating the disc. Great fun and oft demonstrated on RAF medical officer courses.

We can all get flicker vertigo in the right circumstances and unless it occurs in on individual on an unusally frequent basis should not be cause for disqualification.

FOM

[Robbo Jock]

Nick,

With respect to that cartoon in Japan. Apparently, when the cartoon was screened, several hundred kids were taken to hospital. Later that day, the phenomenon was reported on the news and, in a spirit of brave journalism and in the public interest, they showed the offending clip. Full speed, no warning. Surprise surprise, hundreds more were carted off. :rolleyes:

[Bad medicine]

FOM has got it absolutely correct. There is much confusion between true flicker vertigo (a false sense of rotation caused by the strobe effect), and the provocation of seizures in susceptible individuals by strobe lights at suitable frequencies. They are not related.

Cheers,

BM

[clearance]

While working as a Aircrewman on a Bell 412 back in 1988' ( err... "Mobile-45") ;) we were conducting a 'High-line' transfer of a 'Ketch' in the Great Barrier Reef hovering at about 90 feet. The patient was being winched in a Strop/Hypo Strop and when he was at about 65-70 feet the Captain appeared to lose partial control of the aircraft. We began drifting left then right, climbing, descending etc not in an uncontrolled manner but!!!! things were not normal I continued winching in and con the aircraft as best I could (which was not having any effect) the Captain did not respond to any commands, comments or swearing? The Co-pilot eventually took over and we settled down at about 20 feet, approx 150-200 feet (rotor disk) from the Ketch. We recovered the Rescue-Crewman who had the high-line pulled out of his hands (he jumped in and swam over to us when signalled. The Captain was stood down from flying duties, pending several tests. He had the tests and resumed full flying duties subject to flying two pilot ops.

We were all briefed that he suffered from what they called at the time 'Rotor Flicker'. He had had a similar incident during his time in the RAAF flying Huey's.

Cheers

Clearance.

[spinwing]

Certainly I can remember in 1980 when undergoing the initial flight crew medical for the issue of my UK ATPL I was tested for this phenomenon.

Was not tested by the Australian or American Medical Authorities, only asked if I had suffered from or had a tendancy to vertigo,dizzieness or epilepsy.



:hmm:

[Gomer Pylot]

On my final instrument checkride in flight school, in an H13, I got something like that. In holding over an NDB, with the sun above and behind me, I got lots of flicker on the instrument panel on the inbound leg. I became so disoriented that I was having trouble controlling the helicopter. Cleared for the approach, I stayed at holding altitude until the beacon, then went down to PT altitude for the approach. At the MAP, I started climbing, then realized what I had done. The examiner let me fly the approach again, because I obviously hadn't busted any altitudes. :yuk:

BTW, the reason the greenhouse window cover in the B412 is required is because of the flicker vertigo problem with the 4 blades - or at least so I was told.

[Mr Toad]

The use of strobes in cloud or at night when the light is reflected off the underside of the rotor can also induce discomfort in some people - myself included. And I passed the CAA and RLD tests (plus a few others); so is there something that we know of but don't really understand and the medics haven't yet cottoned on to?

[Bad medicine]

It can cause a form of motion sickness due to the mismatch between the false sense of rotation from the strobe effect on the visual system, and the lack of a matching sense of rotation from the vestibular system. That is probably the basis behind the discomfort that you describe.

Cheers,

BM

[Mr Toad]

BM

Yes that makes sense to me.

[Rot8tor]

FOM: Thanks for the enlightening information.

crab: Firstly the French don't drive at normal speeds, and secondly I understood the spaced trees were planted solely to provide shade for the h'mm Army

[Cyclic Hotline]

I seem to recall this as the speculated cause of a Mil-8 accident in New Zealand a few years ago. I don't know if it was ever substantiated, or even how this conclusion might have been arrived at, as all crew were lost in the accident.