Oh, what a beautiful morning
Sluggishness on takeoff that just gets worse
By Greg Marino
It was finally a great morning to fly in northern Wyoming, with severe clear skies and gentle winds after several days of high winds, low clouds, and rain, so I took advantage of it.
It had been a month since I flew my Mescalero (the U.S. Air Force version of the Cessna 172), with the combination of my work in Alaska and foul weather back in Wyoming getting in the way, so I was glad to taxi from my hangar to Runway 24 and to get back in the sky.
Nothing unusual was seen on the preflight, and the run-up was normal, although I did notice the fall in rpm with the carb heat open was less than usual. I didn’t think much of it; after all the rpm did fall, although not to their usual amount. And it was hot with density altitude at 6,400 feet from the field elevation of 3,900 feet, not unusual for summer in Wyoming. The takeoff roll seemed normal, with the expected minor sluggishness at this density altitude, and it did not appear out of the ordinary.
On the climbout the sluggishness became more noticeable, and the rpm slowly dropped. The climbout is not really the time and place for power loss. Runway 24 climbs slowly up rising terrain that is mostly hay fields for a mile or two before encountering the foothills of the Big Horn Mountains. It was time to cut and bale hay, something I had never done, and I did not like the idea of making my first attempt to do so in a 172. But we were still climbing, very slowly, and when I had sufficient altitude and speed, and it was safe to do so, I gently turned toward the downwind for the crossing runway, in control of the aircraft. Thankfully our airfield is not very busy, and I asked the only other pilot on the field at this time to hold for me while I dealt with the issue, and he was happy to do so. As I turned final on Runway 15, the engine started to run rough, with the roughness increasing until I softly touched down, exited the runway, and headed back to the hangar. I taxied slowly back with the mixture lean as is my procedure, and the roughness seemed to slowly get better.
Back at the hangar, my wife came to join me in taking off the top part of the cowling, and we did not see anything amiss. The carb heat control mechanism also felt like it was working normally. My mechanic came over and removed the air intake screen, which showed a flattened duct. When we pulled the bottom half of the cowling off it revealed a two-foot length of wire protruding from the air intake duct hose, with the duct itself having collapsed like a flattened drinking straw. The duct, which looks like the dryer vent ducting in the laundry room, is held open by the spring like tension of the wire. The wire had broken and with the vibrations of flight worked its way out of the hose, which then collapsed, obstructing the engine air intake. In short, the engine was slowly suffocating. And density altitude didn’t help matters either.
Except for the ducting, there was no other damage, and the repair was quick and easy. Run-ups were normal, as was the rpm drop with carb heat, and the first post-repair flight showed normal rpm on takeoff roll and climbout. That first flight, although short, was completely uneventful.
Although collapse of the air intake duct with subsequent blockage of the duct is in the differential diagnosis of power loss on takeoff and climbout, along with sudden activation of the primer, or opening of engine carb heat, the pilot’s primary job is to aviate, protecting those onboard and on the ground as well as possible until a safe landing can be accomplished. Then consider what went wrong. But I wonder if maybe a periodic inspection of the ducting and the air intake screen could go a long way for prevention, as well. It’s pretty easy to do.
Greg Marino is a doctor based in Wyoming and Alaska, where he serves rural native people. He owns a Cessna 172 and a Taylorcraft.
