Because power should be added now and then for the above reasons during simulated emergencies, flight instructors and students are admonished to remember that the gliding distance to the site of a real power-off landing will be shorter than those training glides that are extended by brief additions of 1,500 to 1,800 rpm of power every 30 seconds or so.
Carburetor ice is not the only way an idled engine can be silenced during a long glide, and a long and accomplished aviation career was almost cut off before it began by another cause of idled-engine failure. Only luck kept the 290-hour pilot alive in on a winter day in 1960. Recently the same fellow, now a 21,000-hour working pilot, recalled the fateful day, jogging his memory with a newspaper clipping and an undignified photograph of his airplane resting inverted with its tail dangling from a tree.
It was February 4, 1960, and the then-21-year-old flight instructor and his 22-year-old student had departed from Bedford, Massachusetts, in a Cessna 150 powered by a 100-hp Continental O-200 four-cylinder engine. It was a cold, dry, clear day - not conditions that most associate with carburetor icing. The flight instructor was working for $10 a day, training students for a collegiate flying club. He was paid the same $10 daily stipend regardless of the number of hours flown. He had given 60 hours of instruction total, was working seven days a week, and had soloed his first student the previous week. "I had about 300 hours, and I thought I knew almost everything," he said. The flight had progressed about 20 nautical miles west to the Ayer, Massachusetts, area. "We were in cruise at 4,500 feet when I reached up and retarded the throttle to idle and stated, 'Engine failure.'" The flight instructor did not pull on the carburetor heat.
As he should, the student immediately began looking around for a suitable landing field. This cast light on another ambiguity of the situation: All of the nearby fields were snow-covered, so it was not really possible to tell the good prospective landing sites from the not-so-good alternatives. The flight instructor's intention was to take the maneuver down to as low an altitude as permissible, to let the student see if he really could have succeeded in making an emergency landing. "He found a field and set up a glide. The engine was still at idle with no carburetor heat," the CFI recalled. The student bled off most of his altitude during the glide and "set up some kind of a pattern."
But at this point, the chain of events building up to an accident achieved critical mass. Still fairly far out on a final approach, the student applied flaps - full flaps - much too early. (Readers familiar with the characteristics of the Cessna 150 may wince at reading those words, knowing how dramatically the deflection of all 40 degrees of flaps in the little two-seat trainer steepens its sink rate.)
"I saw we were not going to make the field," the flight instructor recalled. "So I said, 'Well, we are not going to make it. Let's go around.' But it was the first time I had asked for power in about five minutes. The throttle was pushed in quickly, and the engine didn't even respond. It didn't cough or anything. In a matter of seconds, we were in the trees."
In the tops of some stout, 50-foot-tall pine trees, as it turned out. "We went in, turned upside down, we looked at each other, and then it fell to the ground," the flight instructor related. A tree limb punctured the windscreen, but neither of the two occupants was seriously injured. Some loggers in the area rushed to the scene. On seeing that the two aviators were unhurt, one of the loggers joked that it wasn't even safe to be working in the woods anymore.
Reflecting on what had happened, the flight instructor concluded that the prolonged cooling of the engine had created a more insidious problem than carburetor ice, with its typically gradual onset. More likely, the cold ambient air flowing into and around the engine had chilled the induction system to the point where the fuel could not be vaporized for combustion upon the sudden application of full throttle. Here again, regular clearing of the engine, recommended to avoid so-called shock cooling of engine components, might have prevented a vaporization problem. And one pilot looks back and realizes that better engine management of emergencies might have protected him from the solitary accident in a long and diverse flying career.
None of which is to say that these issues are only concerns for pilots engaged in flight training. The combination of low altitude, low power, and a delay in taking the correct preventive measures can strike others as well, as two occupants of a Cessna 172 discovered near Pine Bluff, Arkansas, on January 6, 2001.
According to the National Transportation Safety Board report of the accident, "The pilot was flying the airplane at 1,200 feet agl, looking at the ice damage caused by a recent storm. As the airplane was crossing plowed fields, which were surrounded by trees, the engine rpm dropped to around 1,100." The pilot "pushed the fuel mixture all the way to rich, pulled the carburetor heat control, pushed off the cabin heat, and pushed the throttle all the way forward," with no increase in engine performance. A forced landing was initiated on a gravel road. During the landing roll, the airplane drifted off the left side of the road into a muddy ditch. The airplane nosed over and came to rest inverted. The pilot reported that the loss of engine power was due to carburetor icing." The NTSB concluded that delay in adding carburetor heat was a "contributing factor" in the event.
As for the young flight instructor in the Cessna 150 in Massachusetts: He flew again several weeks later, but he did not resume flight instructing until May of the same year, by that time having undergone a flight examination with a federal inspector and been recertified. I asked him to describe how the entire ordeal changed him as a pilot. He was ready with his answer, and we all know that there are pilots out there who can benefit from the advice without duplicating the lesson.
"Having an accident is a rather traumatic experience. It is interesting that I have never had another once since. It makes you realize, 'Hey, wake up, you have to learn perpetually.'"
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