Cabin Fire Caused by Sun and Canopy?

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dennisvoriespe
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Cabin Fire Caused by Sun and Canopy?

Post by dennisvoriespe »

A few days ago on a sunny noontime day at Cedar City, Utah, I had the canopy fully open on my Diamond Twin Star while I was completing preflight checks for my flight back to San Diego. As I climbed into the cockpit, I noticed an odor much like electrical insulation burning and immediately saw smoke rising from around the left canopy support. As an electrical engineer who does owner assisted annuals, I tried to imagine what part of the electircal system could possibly be shorted and immenently on fire with the master switch off . While I grabbed the fire extinguisher from its bracket beside our two year old strapped in her back seat car seat, I commented to my wife that a cockpit fire was the last thing I wanted to have.

With closer observation, I noticed what appeared to be a smoking white hot half inch diameter area beside the cabin support relief of the carbon fiber instrument cover. I brushed the hot spot with the tip of my left index finger and instantly got a burn. More importantly, I saw the white hot area momentarily disappear and immediately realized that the smoking hot spot was not an electrical issue but the focus of solar energy reflecting from the inside of the gracefully curved canopy - perhaps a portion of the white painted canopy top area. A slight change of the canopy angle relative to the sun that was slightly behind caused the hot spot and smoke to disappear. The hot spot left the black paint slight dulled with a slight dime size depression in the cover.

Lesson: If you have a canopy that opens like the Diamond DA-42, beware of the Sun's location while open...you might end up with a hole burned in your interior or worst case a fire.

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Re: Cabin Fire Caused by Sun and Canopy?

Post by Antoine »

Unbelievable! Thanks for posting.
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Re: Cabin Fire Caused by Sun and Canopy?

Post by Jean »

Absolutely unbelievable. I don't figure how a canopy could behave as an optical lens ?
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dennisvoriespe
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Re: Cabin Fire Caused by Sun and Canopy?

Post by dennisvoriespe »

The canopy was actually behaving as a parabolic reflector.
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Re: Cabin Fire Caused by Sun and Canopy?

Post by Jean »

Wow !
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Re: Cabin Fire Caused by Sun and Canopy?

Post by rwtucker »

dennisvoriespe wrote:The canopy was actually behaving as a parabolic reflector.
Amazing! I would never have thought the canopy could both reflect and concentrate enough energy to reach those temperatures.

Given that the sun is below the zenith this time of the year, what position was it in in relation to the aircraft? I'm having trouble picturing any area on the canopy that would form enough of a reflective parabola to do the job. I can maybe see a weird partial parabola at the front, near the attachments if the sun were someplace reasonably high and aft. I could maybe see a partial refractive lens, also near the front attachments if the sun were fore of the aircraft. To refract and concentrate, there would have to be a variation in the thickness or a random variation in chemical composition of the Plexiglas.

Maybe someone who is looking at their canopy right now can find a possible spot but I don't think a simple reflection with no concentration, even 100% reflection which is not possible, would do the job. The fact that there was a small isolated spot that got hot enough to decompose suggests that significant concentration was taking place.
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Re: Cabin Fire Caused by Sun and Canopy?

Post by Antoine »

I think Dennis mentioned the top central part (painted white)? Makes sense to me... This now and the other horror story with the iphone blocking the stick scare me.
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Re: Cabin Fire Caused by Sun and Canopy?

Post by rwtucker »

Antoine wrote:I think Dennis mentioned the top central part (painted white)? Makes sense to me...
I saw that. I haven't had my morning coffee so that may explain everything . . . but can you sketch out the position of the sun with respect to painted area that would reflect and concentrate a beam that narrow?
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Re: Cabin Fire Caused by Sun and Canopy?

Post by dennisvoriespe »

I understand the healthy skepticism about this incident of nearly a week ago. If I hadn't seen it myself, I too would questioned it. I had my family belted in and ready to fly, otherwise I would have taken a little time to investigate the exact location of the reflecting area. It would have only taken a minute of moving my hand around the inside of the canopy while observing the white spot and I regret not taking time to do that. The good news is that the question of culprit canopy area can be resolved both analytically and empirically from an abundance of available information.

First some background: I've spent my career as a degreed and licensed Professional Engineer in private practice doing electronic and mechanical R&D and forensic engineering including development of both semiconductor and thermocouple based temperature probes for production. Early in my engineering career I wrote a small book on the subject of solar to thermal conversion so am familiar with the forces at work and their analysis.

This incident happened on a clear April 19th day within a few minutes of noon daylight savings time at Cedar City, Utah having a North latitude of about 38 degrees. At a glance, I estimated the Sun to be slightly starboard as would be expected and about 20 degrees rear of the plane. At this time of year at Cedar City, the sun is 2/3 of its way from a low of about 61 degrees South to about 15 degrees South putting it at an estimated 30 degrees South. The DA-42 was parked aimed north within a +/- a couple degrees of perpendicular to runway 085/265. When correcting for magnetic variation of 12E degrees, this aimed the underside of the raised canopy at about 187 degrees true South. The spot was less than 1/2 inch in diameter and appeared fairly round indicating that is must have come from a corresponding effectively dish shaped area on the canopy. Given the foregoing facts including the spot location relative to the canopy, knowing that the angle of incidence is equal to the angle of reflection and the raised angle of the canopy, it should be relatively easy to estimate the area of the inside of the canopy acting as a reflector. Next time I'm at the hanger I'll take along a digital inclinometer, protractor and a laser pointer and report my findings.

In terms of energy and amount of area involved, the sun would have been delivering 80 - 90 W/Sq. Ft. to a surface normal to its rays. For purposes of argument, consider that a 1/2" diameter circle has an area of 0.20 sq. inch. For a 100X focus in solar intensity with a combined theoretical 50% canopy reflectivity and black surface absorptivity, the required parabolically shaped area would only need to be about 7" in diameter delivering 5W to the white spot. With the already normally solar heated ambient temperature of the entire cover top near the threshold of pain of 113F, this certainly would have been sufficient to create the smoke and carbon composite softening observed.
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Re: Cabin Fire Caused by Sun and Canopy?

Post by rwtucker »

dennisvoriespe wrote:I understand the healthy skepticism . . . For a 100X focus in solar intensity with a combined theoretical 50% canopy reflectivity and black surface absorptivity, the required parabolically shaped area . . .
Fantastic briefing Dennis! :thumbsup:

No skepticism here. I'm just trying to visualize the location of that rough-hewn parabola. Where on the white area do you estimate is was such that it produced a 100X focal point at the location of the melt down?
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