Comments
This inspection is a giant PITA, exacerbated by inexplicable errors in a poorly written section of the DA40 AMM. Unless your aircraft is amazingly pristine, plan to spend at least one full day chasing poor or intermittent connections.
Issues with the AMM include:
- The AMM specifies that the low resistance bonding measurement “test current must be approximately 2 amps.” However, 2A test equipment is hiding with unicorns, while 1A test equipment is included with the Diamond bonding test kit. Diamond’s explanation is that “1A is approximately 2A.”
- The AMM says that “the airplane must be in a serviceable condition during the tests, except that the engine cowlings must be removed.” Apparently having *every* access port open, with the horizontal stabilizer shroud and stainless belly pans also removed is considered “serviceable condition."
- The purpose of the bonding test should be to ensure that 1) the resistance across the airframe is sufficiently low to dissipate static, and 2) verify that critical lightning protection is intact. But the difference between 10 milliohm and 15 milliohm (or even 50 milliohm) is not material. Any of these values are sufficient to ensure excellent static dissipation and lightning protection. Unfortunately, the values listed in the AMM are (IMO) nominal values useful for troubleshooting poor connections. But they have no engineering basis for pass/fail.
This would not be a problem, except that remediation for out-of-“spec” values involves disassembly. Potentially of nearly the entire airframe!! In preparation for this inspection I consulted with very experienced Diamond-certified maintenance shops and several owners who had completed the 1000 hr inspection. I heard horrifying tales of major surgery in pursuit of the seemingly arbitrary limits shown in the AMM. People described complete dis-assembly of the tail section, removing the forward spar pins to “wiggle” the wing, full wing removal, etc., all to tweak a resistance value slightly out of spec. The safety risk seems extraordinarily large for such a minimal improvement. - The AMM is completely silent about bonding deficiency remediation. The comparison with (for example) Laminating (51-20) is striking. See "F" below for Diamond's recommendations via email.
- Plan to have the 1000 hr inspection coincide with your annual. The required level of disassembly is quite similar, and this will save you a lot of time.
- Discuss with your IA how you plan to handle the tolerances in the AMM, before you dive into the tests.
- Take great care when checking resistance around the fuel drains. A passing test generates a significant spark, which could be disastrous around vaporized fuel.
- Be very careful with the static wicks. The pot-metal is extremely brittle, easy to break, and the wicks (Dayton-Grainger 16630) are ~$50 each. DAMHIKT.
- Note that unlike every other static wick on the aircraft, one of the two screws holding each of the horizontal stabilizer wicks is actually a different pitch (US standard, vs. metric). The threads look very similar, but are definitely different, and easy to strip.
- Via email, Diamond provided the following suggestions for remediation:
Personally I disagree with the recommendation about contact cleaners (I asked specifically about Deoxit), but this is not surprising.Diamond Aircraft support wrote:The vast majority of the time a bonding issue can be resolved by simply loosening and tightening a fastener. Sometimes bonding connection points may have to be disassembled, inspect for corrosion and cleaned with suitable medium, sand pads or sanding paper as required. Reapplication of Nycote 7-11 Blue or other suitable commercial protective lacquer for electrical connections sealing. If the corrosion is too deep then the item would have to be replaced. We do not use any contact cleaners or other liquids such as Deoxit when assembling the connection points. - In the US, Diamond charges $125/week for the test kit, starting from the date you receive the box. You are also responsible for ~20 lbs return shipping to Indiana.
Chris
