Proficient Pilot

10 knots below

May 1, 2006

Retired TWA captain Barry Schiff has been flying for more than 50 years.

Several months ago, I needed to rent an airplane in the Seattle area and arranged for a checkout in a Cessna 172S with a flight school at Boeing Field/King County International Airport.

I was asked during the checkout to perform a sampling of maneuvers from the Practical Test Standards for the private pilot flight test. While rolling out of a requested 720-degree steep turn at 6,000 feet agl, the instructor retarded the throttle, and announced, "Engine failure!" This is not an unexpected challenge at such a time. I confess that I did not have an emergency landing site in mind but soon saw what appeared to be a suitable landing strip almost directly beneath us. I reduced indicated airspeed to 58 knots, halfway between the power-off stall speed (48 knots) and the speed for best glide (68 knots), and then simulated an attempt to restart the engine (check ignition, fuel, and so forth).

The instructor commented that my speed was 10 knots low and asked if I knew the speed for best glide. I replied that I did but that it was not appropriate to use at a time like this. He looked at me as though I were some kind of nut case and told me in a disapproving manner that we would discuss this on the ground. "Head back to the airport and set up for a short-field landing."

After I chocked and tied down the Skyhawk, the young CFI led me into an empty classroom, motioned for me to sit down, handed me a copy of the airplane's information manual, and asked me to read aloud the procedure to be used following an in-flight engine failure.

"The first step," I read, "is to establish an indicated airspeed of 68 knots." Both of us knew that this was the airspeed to be used to achieve best glide. I now had to be tactful unless I wanted to go elsewhere to rent an airplane.

"Sir," I asked in my most respectful tone, "isn't the purpose of the best glide speed to maximize glide range?"

"Of course, it is, and you flew 10 knots below that speed."

"Yes, that's right," I said. "That's because I did not want to fly for maximum range. Shortly after you pulled the throttle, I found a place almost directly beneath the airplane that I considered suitable for an emergency landing. As a matter of fact, that grass strip looked as though it might have been a privately owned airport at one time."

"So exactly what is your point?" he inquired, obviously beginning to lose patience.

"Well, sir. I did not need to maximize glide range; we were already where I wanted us to be. Instead, I wanted to minimize our sink rate and maximize our time in the air. Consequently, I opted to fly at an airspeed close to that used to achieve minimum sink."

It seemed as though he wanted to tell me that he didn't know what in the heck I was talking about, but he held back. "Go on; I'm listening."

I went on to say that although the normal (or best) glide is the only type of glide taught to most pilots and is the only one mentioned in pilot operating handbooks, there is another type of glide that can be equally important, depending on the need of the pilot at the time. It is called the minimum-sink glide, something about which glider pilots are intimately familiar. It is used when glide range is unimportant but when the pilot would prefer instead to have additional time in the air; time needed perhaps by a power pilot to attempt a restart, brief his passengers, or simply gather his wits and prepare for the emergency landing.

Glider pilots use "best glide" when flying from one thermal to the next but use "minimum sink" when circling within thermals to maximize climb rate within them.

These two types of glides, minimum sink and maximum range, are loosely analogous to the two types of climb speeds used by power pilots. The best rate-of-climb speed is used to gain maximum altitude in minimum time, while the best angle-of-climb speed is used to gain maximum altitude in minimum distance.

For example, a Cessna 152 has a 725-fpm sink rate when flying at its normal glide speed of 61 knots. From an altitude of 8,000 feet agl, for example, such a power-off descent would take 11 minutes. But if airspeed is reduced to about 45 knots, sink rate is less, say 600 fpm, and this results in a 13- minute 20-second descent. In other words, flying at the minimum-sink glide speed provides an additional 2 minutes 20 seconds of flying time in this example.

Although this postpones contact with the ground, one must be careful not to carry things too far. When at least 1,000 feet agl or when turning might be required, a pilot should resume the normal and faster glide speed to increase maneuverability and fly a reasonably normal glide path to touchdown.

The problem is that although glider pilots are provided with the speed for minimum sink, there is no way to extract it from an airplane's POH. As a reasonably good rule of thumb, use a speed about halfway between best glide and clean stall, and that will be close enough for government work.

Thankfully, the instructor signed me off and approved my rental of the Skyhawk.

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