Strobe flash rates

[Genghis the Engineer]

I wonder if one of you clever medical chappies could help out a slightly confused engineer.


It's been set in stone forever that strobes on aircraft have to flash at between 40 and 100 flashes per minute. Now my (limited) understanding is that this is considered optimum (a) to give maximum visibility and (b) to minimise any risk of an observer (or the pilot if the lights are reflected back of, say, close cloud) suffering a strobe induced fit.


But, I've got a problem with somebody wanting me to approve a mod to fit strobes that flash at about 16 per minute - which is a great deal slower. Sooner or later I'll probably have to go to the CAA about this, but in general terms I'd like to know

(1) If this sort of rate offers a safety problem in terms of effects upon a human observing them, and

(2) If this sort of rate significantly degrades the chances of it being spotted by another aircraft.

Can anybody help?

G

[Lou Scannon]

Great to know that engineers look at every aspect of a problem!

You may be right that the approved flash rate is the optimum one for both conspicuity and avoiding epiletic attacks. But these thoughts are immaterial.

If your firm wanted you to fit blue lenses to the port nav light, would you worry yourself whether blue was better or worse than red for being seen?

[Flyin'Dutch']

But, I've got a problem with somebody wanting me to approve a mod to fit strobes that flash at about 16 per minute

Why?

[Genghis the Engineer]

Flyin'Dutch' Said... Why?

Simple, we've got a rulebook that says what will be approved and what won't. It's simple and clear on the point that 40-100 flashes per minute will be approved, and tells me nothing about other flash rates. So if it flashes at 39 or below, or 101 and above, I need a good reason to justify the approval. This is when we start to really earn our living as Gingerbeers - having to think and analyse rather than just follow somebody else's rulebook.



Lou Scannon Asked... If your firm wanted you to fit blue lenses to the port nav light, would you worry yourself whether blue was better or worse than red for being seen?

Firstly I'm not a maintenance technician (except on my own aeroplanes and as a favour to a few friends) I'm a certification Engineer. It's my job to decide if something can be approved or not - I'm responsible (like any other design signatory) for this to the authority.

Secondly, the Engineering Council "Code and Rules of Conduct" (my office, top left hand shelf) is a similar document to the old Hypocratic oath (or whatever the medical chappies sign up to these days). It defines a clear code of engineering ethics that require me to consider every possibility, and ensure the highest possible levels of safety and legality. So yes, it would worry me.

Also I fly these things as well, so have a vested interest in the whole subject :cool:

And finally, it's far more fun to be really thorough in a job than just apply minimum standards and sign it off.

G

[Flyin'Dutch']

Nooooooooooooooooooooooooh,

My why was not to question the rule book! It was WHY does anyone want to modify the flash rate on his/her mount!?!

FD

[Genghis the Engineer]

Unfortunately they seem to have bought an off the shelf unit that flashes at that rate, before discovering that this was a problem (to be fair, you can't really expect the average private aircraft owner to know this is important).

It's an irritating fact of modern life that there are a lot of people in the GA world who sell gear and then leave their customers to arrange approval or find out if it can be approved at-all.

G

[Flyin'Dutch']

Send it back as it is unsuitable for the purpose for which it was procured.

[Genghis the Engineer]

Please believe me, I wish he would.

Sadly "it's a stupid idea, and there are much better ways of doing this" isn't always something I'm allowed to say.

G

[airborne_artist]

Tell the owner that it will take 6 months to get CAA approval, and until then he won't be covered by his insurance!

[slim_slag]

(1) If this sort of rate offers a safety problem in terms of effects upon a human observing them, and

Doubt it, what frequency did the CAA use to try and trigger strange waves on the EEG for class 1 issuance? Probably less than 20Hz but greater than what you are proposing.

(2) If this sort of rate significantly degrades the chances of it being spotted by another aircraft.

Doubt it, I think it's the duration of the flash itself (short) that causes the attention to be caught. From distant memories of chopping up brains we use some primitive pathway in the brain, ISTR that people who are totally blind from destruction of their visual cortex in the back of their brain can "see" short flashes (google "superior colliculus" I think).

[InFinRetirement]

I agree with FD. I would definitely tell him that it is an unapproved type and that you cannot sign it off. You wanna risk a confrontation with the CAA. Tell the guy to get real. He can't have it - and he should have gotten one that could be fitted.

One final thing G. Also tell him that if he wants to break the law as to CAA requirements you MUST report it to the authorities. That is what your authority says does it not?

How can he KNOW what 16 flashes my do. Has he had an EEG btw?

[Loose rivets]

How far will he travel between one flash and the next?

[aviate1138]

Presuming that the aircraft concerned will only ever be VFR does it actually matter? Surely that rule is for IFR regimes?

Aviate 1138

[ATPMBA]

From ATPL GS Human Performance & Limitation states that bright flickering lights can cause epileptic type fits in susceptable individuals.


Flashes of between 5 to 20 Hz (cycles per second) can cause it.

A 4-bladed helicopter with an with an RPM of 240 can produce a flash of 16 Hz.

[Johe02]

The flash rate was judged to be the optimum to get you noticed and not to be confused with aerials/towers etc.

Also, you should switch it off in cloud to avoid any flicker induced disorientation. .

:ok:

[Cyclic Hotline]

Why would anyone want to install this light? Did he buy it "cheap" at a World War 2 close out sale?

Surely the simplest way to do this is to purchase a TSO'd item off the shelf, that already meets all the requirements. Is this unit an actual aviation item, or something that he wants to modify for the application?

I'd give him a copy of the appropriate TSO, (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgTSO.nsf/0/54887454079D27D786256DC60069639F?OpenDocument) FAR (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/CE9BA4778329019D85256613006E1730?OpenDocument) and a list of websites where he can get a cheap ready to install unit.

But then, you know how people are. :)

[Genghis the Engineer]

If anybody's interested, here's the story so far.

(1) Chappie bought unit, fitted it, and wanted me to arrange approval (aarrghhh, why don't people ask first).

(2) I come along, find the problem with flash rate - but so far as I can tell it does nothing else to cause a problem.

(3) Attempts to contact the strobe manufacturer prove.... that they've gone out of business (whether this is down to a failure to sell kit that has a chance of being approved I've no idea).

(4) I finally tracked down somebody at CAA who understood strobe approvals and we talked through it. It seems that the 40 fpm minimum in all the regs is to give a high-prob of even a fast "fighter pilot style" lookout sweep picking up the flash - the risk of an epileptic fit seem to occur at rates above 100fpm; hence the 40-100 range. So the conclusion is that we either

(a) bin it, or
(b) placard it as not a high-viz device.

So, we'll probably go for option (b) in day-VMC only, just so long as we can prove it doesn't endanger the aircraft or create unacceptable RF.


Certification's a great game - in theory designers will always ensure that what they are designing and fitting will be approvable. In practice far too often they design, build, fit, and then blame the certification engineer for making approving it difficult for them !

G

[criticalmass]

If possible, get an avionics tech to open the control unit and adjust the flash-rate timing components (usually some sort of simple oscillator like a relaxation oscillator or similar) to increase the flash-rate to somewhere within the approved range.

Strobes are not complex electronically. An inverter or DC-DC converter, rectifier, voltage multiplier, dump capacitor across the tube itself, trigger-transformer and trigger cap charged up by a charging circuit and usually an SCR and our little oscillator to fire the trigger cap through the trigger transformer to cause the tube to ionise and therefore discharge the dump capacitor straight through the tube - usually shutting down the inverter which then starts up again, charges up the dump capacitor and the whole cycle repeats.

The timing components of the oscillator circuit determine the flash-rate, as long as the time constant for the dump capacitor is short enough to allow full charging at a higher rate. Usually this is not a problem.

Might be the easiest solution.