Plane & Pilot
Tuesday, September 3, 2013

The Lowdown On Go-Downs

Descents should be more than simply throwaway methods of losing altitude

I was cruising high for a normally aspirated twin, scoring a ground speed of 200 knots at 13,000 feet in a freshly restored Cessna 310R. As I looked down at the gently rolling Atlantic, the first of the Canary Islands came into view. I had departed early from St. Johns, Canada, refueled in Santa Maria, Azores, and now, I was sneaking up on my second and last stop of the day, Tenerife, some 2,000 nm southeast of Newfoundland.

As usual, the Canaries were shining in the sun, the Bahamas of Europe, only 100 miles off the coast of the Western Sahara, Africa. It was almost embarrassing to admit I was tired in such a comfortable airplane, even if the day had stretched to nearly 10 hours of flying.

I had picked up a passenger in Bangor, Maine, the day before—John Thorson, an A&P mechanic and former charter pilot. Thorson was definitely the old pro; he was so comfortable in an airplane that he had slept for most of the last 2,000 nm. He woke a few minutes before Canaries control approved my descent into Tenerife.

Cleared down to 5,000 feet, I eased the nose over and reduced power slightly to keep speed in check. Passing through 10,000 feet, I started pushing the mixtures forward on the normally aspirated Continentals. That brought Thorson to life.

"Why are you advancing the mixtures?" he asked.

"Well, John, I'm fattening up the mixture for lower altitude," I answered.

"Should be going the other way," Thorson mumbled quietly, as if he was embarrassed that he might offend the captain.

I questioned his suggestion, and he said, "Look, you've got the power back to about 55%, so you can't detonate the engines with any reasonable mixture setting. Why not lean the mixture slightly, rather than richen it to help keep the cylinders warm during the descent and avoid shock cooling?"

That was a bit of mechanical wisdom I had never considered, but it made perfect sense. Like most pilots, I was in the habit of advancing the mixtures rather than retarding them as I descended.


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