Friday, September 25, 2009

Landing Light Switches

Back on May 11th, Cessna issued a Service Bulletin, numbered SEB09-6 on the subject of landing light, taxi light and beacon switches in the legacy 100-series aircraft (including, of course, the 182's like N631S).

There have been reports of these switches failing in the field, causing overheating and smoke in the cockpit. Burned insulation has been found on the wiring terminals to the switch and on at least one occasion, there was a small fire in the cockpit. The problems are being attributed to contact erosion leading to irregular arcing and eventual failure of the switch. Apparently, some laboratory testing was done and it showed the contact erosion occurring after approximately 4,000 cycles of operation.

Service Bulletin SEB09-6 specifies the following:

  • Inspect to determine time-in-service for the switches.
  • Write the month and year of the initial installation on switches that have been in service for less than four years.
  • Replace of switches that have been in service for four or more years, with the month and year of the installation written on the new replacement switch.

SEB09-6 is classified as "Mandatory". Of course, for we who fly under Part 91 rules, Mandatory doesn't mean Mandatory. The FAA has looked at the situation and issues a Special Airworthiness Information Bulletin (SAIB), number CE-09-42 dated 24 July 2009, stating that this is not worthy of issuing an Airworthiness Directive (AD), but that maybe we ought to do what SEB-09-6 suggests within the next year or so (or 400 hours if that comes first).

With all of that going on, I've been having some issues of late with the landing light and taxi light switches on N631S. I would move one or the other to the "ON" position and the corresponding light or lights wouldn't respond - until I cycled the switch a time or two. Now, of course, you don't really need a landing light, but with night-flying season approaching I decided it would be good for the lights to work reliably. So I asked my friends at Three Wing (at KBDR) to replace the two switches (the beacon switch has been no trouble at all so we are leaving it alone).

Here is a photo of the switches that were removed:

There do appear to be some arc tracks on the back side of the switches. The thing between the two switches is a diode bridge that lets the landing light switch energize both the landing light lamp and the taxi light lamp. If you click to enlarge you'll see that, of two diodes installed in parallel, one is burned through. The circuit has been running on a single diode for an indeterminate time - not that there's anything wrong with that.

The old switches are Carling p/n S2160-1. They are marked as rated for 5A 250v, 10A 125v, 1/2 HP 125-250v. The replacement switches are Carling p/n TA201 TW-B. They are rated for 10A 250v, 15A 125v, 3/4HP 125-250v. (Of course none of the ratings discuss 14vDC service.) At least Cessna has now specified a "beefier" switch.

Now, about that "replace every four years" notion. How does that make any sense at all? The only data seems to be that the old switches get in trouble at about 4,000 cycles. Let's suppose that the new, more better switches don't improve on that. Then let's suppose N631S and I go flying three times/week (we don't). Let's also suppose that on every flight we cycle the switches twice, once for preflight and once "for real". (Obviously, not every flight is a night flight so that's conservative.) That gets me to about 300 cycles per year - it'll take about 13 years to accumulate 4,000 switch cycles. Perhaps I should plan to replace those switches in about ten years.


Ferg Kyle said...

As a Canuck, I read the original report and recommendation - and your assumption seems adequate enough. My question is: If we share the 12 or 24 volt DC empire with motor vehicles, why cannot the makers specify a DC rating/ This is not aerospace surgery....

Frank Van Haste said...

Hi, Ferg, thanks for reading...

In answer to your question, I guess it's because if they specified 14/28vDC ratings the switches wold become airplane parts and the cost would need to go up by 1 order of magnitude.

Fly safe(ly),


Anonymous said...

Our 1956 172 switch failed this week, with smoke in the cockpit, after about 7500 hrs. Manny Block