Retired airline captain Barry Schiff regularly practices engine-out procedures.
To paraphrase a timeworn adage, "The only thing you can expect from the remaining engine of a light twin (following an engine failure) is that it will take you to the scene of the accident." Such a cynical attitude stems from the poor engine-out performance of many light twins. The worst performing modern twin, the Piper Seminole, has a single-engine service ceiling of only 4,100 feet msl.
A conclusion drawn from this is that an engine failure over mountainous terrain is likely to result in colliding with a contour line. Such a predicament, however, is rarely that dire. By drifting down gradually, a pilot is afforded more time and distance than he might imagine. Except in extreme cases, a crippled twin can hobble to an airport and land even when the airport is above the airplane's single-engine ceiling.
A published ceiling usually applies only when at maximum-allowable gross weight, an improbable condition considering fuel burned en route. An aircraft weighing less often can maintain an altitude twice as high as its published engine-out service or absolute ceiling.
Some years ago I was contracted to flight test the driftdown characteristics of the Seminole to determine what a pilot flying over the Rockies, for example, could expect following an engine failure. (Better performance could be expected from almost any other light twin.) While cruising at 14,000 feet, I throttled back the left engine, feathered the propeller, and allowed the airplane to decelerate to the best engine-out rate-of-climb speed. Although the airplane obviously would not climb, V YSE results in the minimum sink rate during driftdown.
The first 1,000 feet of altitude loss took 3 minutes and 36 seconds, resulting in an average sink rate of 277 fpm. Descending from 13,000 to 12,000 feet required almost 6 minutes, an average sink rate of only 167 fpm. The descent profile flattens during descent because of increasing air density and engine power. It took an hour to descend to 8,000 feet, where sink rate was only 44 fpm. The Seminole leveled at 7,600 feet, almost twice as high as the published engine-out service ceiling of 4,100 feet.
Still-air range during descent was 104 nautical miles, and average sink rate during the 5,000-foot loss was less than 100 fpm. No matter where in the 48 states such an engine failure might occur, the Seminole would almost always be within driftdown range of an airport, indicating that once any light twin has climbed to minimum safe altitude, an engine failure above the service ceiling rarely dictates the need to land off-airport.
If driftdown begins over high-rise terrain, the pilot obviously should head toward lower elevation and, hopefully, a suitable airport. To keep sink rate at a minimum, make only shallow turns and hold a steady pitch attitude. If an autopilot is available, use it to reduce workload.
Once the actual absolute ceiling is reached and maintained, the airplane continues to burn fuel and get lighter. Unless power is reduced, this results in an airspeed increase that can be used to drift up (at V YSE) to regain altitude.
An engine-out approach and landing while drifting down does require a cool hand on the yoke but — believe it or not — is not that much more demanding than approaching a low-elevation airport.
If an airport can be seen, it probably is within range even if an initial appraisal indicates otherwise. Do the math, and you will see that the single-engine descent from 12,000 to 11,000 feet in the Seminole results in an effective "glide ratio" of 71 to 1, considerably better than could be expected from the world's most efficient sailplane. The descent from 10,000 to 9,000 feet results in a glide ratio of 128 to 1, equivalent to a descent gradient of only 0.4 degree.
So, yes, if an airport can be seen, you probably can land there no matter how distant it appears to be. To be certain, however, watch the airport carefully from afar. If the runway moves up with respect to a point on the windshield, you might not make it. Consider that during driftdown, the descent angle becomes shallower (and eventually becomes horizontal). This might confirm the capability to reach an airport previously rejected. If and when the runway moves down with respect to a point on the windshield, you've got it made.
The approach to such a high-elevation airport must be executed very carefully because a missed approach is probably impossible. Fortunately, the maneuver does not require too much fancy footwork. If possible, establish a final approach that is at least 3 miles long and descend on a normal, 3-degree approach angle. Because the engine-out twin has such an outstanding glide ratio on one engine, you should not have a problem even if the aircraft inadvertently dips below the slot. Just maintain V YSE and maximum power to recapture the
glide slope. A normal approach, after all, is a 3-degree slope, while a single-engine twin can be held to a slope of less than 1 degree. Eventually, the descent path of the aircraft will merge with the normal approach slot.
Is a twin safer than a single in the event of engine failure? I think so, but only when flown by a pilot who rigorously maintains proficiency in engine-out procedures. Otherwise, he probably is better off in a single.
Visit the author's Web site ( www.barryschiff.com).
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