I zipped out to the airport today early, arriving around 7:30 AM, wanting to get a quick flight off before a cold front arrived. There was already about five knots of crosswind, which was a nice step up from yesterday’s essentially calm winds. I did a bit of cruise performance testing, then an aperiodic banging sound started. The sound didn’t change when I selected each ignition system OFF - ON, nor did the engine instruments show any anomaly. But, banging sounds are not what you want to hear, so I slowed down and headed back to the airport. 30 minutes airborne.

After I got back to the hangar, I took a close look over the whole aircraft, and found that the rubber wing root fairing seal had come loose above the left wing root.

I’ve been watching an anomaly with the prop governor. In the handful of flights I did after the prop change, but before the car accident, I noted that the max rpm would decrease about 100 rpm as the engine got up to temperature. The rpm would be close to 2700 rpm for the first take-off, but it would slowly decrease about 100 rpm over the first 10 minutes of the flight. I contacted Aero Technologies tech support, and they suggested I check for internal leaks in the prop governor oil system as described in Lycoming Service Instruction 1462A. SI 1462A calls for the prop governor to be removed, and a special cover plate to be installed in its place. The cover plate has fittings allowing the oil passages and external oil line going to the prop to be pressurized with an air compressor, using a differential compression tester. The input pressure is set to 40 psi, and the pressure downstream of the orifice in the differential compression tester must reed between 6 and 35 psi (this pressure is a measure of the amount of leakage in the system - lower pressure means more air flowing through the orifice, which implies greater leakage downstream).

I didn’t want to remove the prop governor yet, as it is a big job. So, I disconnected the oil line at the prop governor, and did the test from there. That allowed me to check for leakage downstream of the oil line - i.e. up at the nose of the crankshaft. With the warm, I got a reading of 12 to 15 psi, which is in the acceptable range. That is excellent news, because a problem in that area would require the engine to go back to the overhaul shop.

I discussed my prop governor issues with Andrew, a local Pitts Special pilot today. His three aircraft have MT aerobatic props too, but his prop governors are not the same as mine. He sees about a 50 rpm decrease in max rpm as the engines comes up to temperature. He suggested that perhaps I should defer digging too deep until I had switched over to my normal oil - at the moment I’m running Phillips Type M 20W-50 mineral oil. Andrew thought that perhaps the symptoms might change when I changed oil types.

I noted during this flight that the max rpm was not directly related to oil temperature as I had originally thought. I can vary the oil temperatue by opening and closing a door in front of my oil cooler. I noted max rpm vs oil temperature as the engine came up to temperature in the early part of the flight, then later on I varied the oil temperature and compared max rpm to the earlier values. I noted that even at the same oil temperature, the max rpm was lower later in the flight. This suggests that my earlier theory of excessive internal oil leakage that allowed greater leakage as the oil thins out might not be the winner.

I checked the travel of my prop control, and noted that the prop control in the cockpit was very close to its stop at the throttle quadrant when the control arm on the engine hit the max rpm stop. I wondered if somehow differential expansion of the prop control cable and its sheaf as things warmed up ahead of the firewall might possibly cause the prop control to hit the stop the cockpit before it hits the stop at the prop governor. I adjusted the cable at the prop governor to give a bit more margin to the stop in the cockpit. We’ll see on the next flight if that makes any difference.