If you shut down an engine above the airplane’s single-engine absolute ceiling, even at blue line speed in a zero-sideslip condition, the airplane will descend. This is called “drift down,” a condition where the airplane gradually descends to the single-engine absolute ceiling at a slowly decreasing rate of descent. Recall that the single-engine absolute ceiling is the altitude at which the airplane can barely hold altitude on one engine if it is flown to its optimal best performance under current conditions of airplane weight, maximum horsepower mixture control on the operating engine, and density altitude.
Single-Engine Service Ceiling
The more commonly cited single-engine climb performance limitation is the single-engine service ceiling. Like many sources, Section 10, Safety Information, of the Beech Baron Pilot’s Operating Handbook (POHs) tells us:
The single-engine service ceiling is the maximum altitude at which an airplane will climb at a rate of at least 50 feet per minute in smooth air, with one engine inoperative. The single engine service ceiling chart should be used during flight planning to determine whether the airplanes, as loaded, can maintain the Minimum En Route Altitude (MEA) if IFR, or terrain clearance if VFR, following an engine failure.
Beech publishes the single-engine service ceiling for the normally aspirated 1961 Baron as 7600 feet pressure altitude at maximum gross weight on a standard day. In 58Ps and 58TCs, the single-engine service ceiling is 13,400 feet pressure altitude on a standard day at maximum gross weight, according to Beech. Cessna’s early-model 310B has a similar single-engine service ceiling, 7700 feet at maximum weight on a standard day. The turbocharged T310R’s single-engine service ceiling is 17,200 under worst-case weight and standard air pressure. The turbocharged but heavier Cessna 421’s single-engine service ceiling is 14,900 feet. In general—subject, of course, to your specific airplane’s Pilot’s Operating Handbook (POH) or Airplane Flight Manual (AFM) data—normally aspirated light twins will have a single-engine service ceiling in the 6000- to 8000-foot range, while turbocharged light twins will be able to climb at 50 feet per minute under optimal conditions up to the mid to high teens.
Single-Engine Absolute Ceiling
What’s the relationship between single-engine service ceiling and single-engine absolute ceiling? Most manufacturers do not publish a single-engine absolute ceiling for their multiengine products. It’s reasonable that the single-engine absolute ceiling will be a little bit higher than the single-engine service ceiling, whatever difference in altitude it takes to gain enough additional power to climb that last 50 feet per minute. In normally aspirated airplanes, it’s likely not much higher.
Since the single-engine service ceiling of the turbocharged twins cited here is fairly close to their engines’ critical altitude, that is, the altitude at which the wastegate is fully closed and above which the turbocharging system cannot make up for air pressure lost in the climb, the change in single-engine altitudes is probably not that much different for the turbocharged twins. For all practical purposes the ability to climb at 50 feet per minute if you do everything absolutely right means the airplane essentially is capped at that altitude. For the remainder of this discussion, we’ll use the more commonly known and more easily referenced single-engine service ceiling as the highest the airplane can go on one engine at maximum gross weight under optimal circumstances.
Single-Engine Drift-Down
If an engine fails above single-engine service ceiling, the airplane will gradually descend to the maximum single-engine altitude before you’ll be able to maintain level flight on one engine—the altitude to which it will “drift down.” Maintaining VYSE (“blue line”) speed as adjusted for weight results in the least rate of descent as your power-challenged twin drifts down.
The POH/AFM describes the technique necessary to obtain the highest drift-down altitude. For example, the Baron 95-A55 POH lists these factors (figure 1):
- Power (operating engine): Maximum continuous
- Gear up
- Inoperative engine propeller: Feathered
- Flaps up
- Airspeed: VYSE
Importantly, obtaining maximum single-engine altitude requires you to lean the mixture for maximum horsepower for that altitude. Although the standard engine failure procedure has you start by advancing the mixtures to Full Rich, after securing the failed engine, you’ll need to lean the working engine for maximum horsepower—about 80ºF rich of peak Exhaust Gas Temperature, which typically results in target EGTs in around 1250º-1350ºF, or Turbine Inlet Temperature in the 1500F – 1600F range in most turbocharged twins. Use the same EGT/TIT targets or “per side” fuel flow targets you use for cruise climb, and you’ll be at about maximum available horsepower on one engine.
Homing in on the topic of this discussion—drift-down. This is what you need to do to maintain the highest possible level flight altitude on one engine. Lose an engine above this altitude, and the airplane will slowly descend to this altitude. Try to keep it higher, and the airplane will slow down, drag will increase, and the airplane will descend even more—not to mention the threat of slowing into a single-engine stall or VMCA condition.
But Weight, There’s More
The single-engine service ceiling becomes much higher with even relatively small reductions in airplane weight. A quick comparison at standard temperature in that example A55 Baron shows some rather dramatic effects (figure 2). A roughly 7% decrease in airplane weight, from 5400 pounds to 5000 pounds for example, results in a nearly 23% increase in single-engine service ceiling—from 7300 feet to 9000 feet. Even lighter is even better—a 9% weight reduction from 5000 pounds to 4500 pounds generates a 33% higher single-engine service ceiling, increasing from 9000 feet to 12,000 feet.
Check the POH/AFM for the twin you fly to see just how much better single-engine performance improves with a reduction in weight.
Using What You Know What are some ways to use this information?
- Fly as light as possible. Carry enough extra fuel to have a very healthy reserve, but don’t overdo it—if you have plenty of options with less fuel on board, don’t top off before takeoff. Give yourself the best option of single-engine flight (as well as single-engine climb capability).
- In mountainous areas, pick a route of flight that gives you the greatest terrain clearance in case of engine failure. “Direct To” may not be the best option over mountains. Start a turn toward lower terrain as soon as possible if you have a loss of engine power.
- If overflying an area conducive to ice, select a route that allows you to level off at the single-engine service ceiling above the icing layer or one that permits you an unrestricted descent through the icing layer at the Minimum Ice Penetration Speed or higher and enough altitude to level off outside of icing conditions—this assuming, of course, you’re flying an otherwise fully functional “flight in icing conditions”-certified aircraft.
- If you’re taking off from an airport near or above your airplane’s single-engine service ceiling, realize that engine failure after takeoff probably means making an off-airport landing on the best available surface within drift-down range. Turns will substantially increase the rate of descent in drift-down, so manage the risks of these takeoffs the same as if you were flying a single-engine airplane—land at the best option nearly straight ahead of the airplane unless you have plenty of altitude to maneuver while maintaining VYSE.
Determine your weight-adjusted single-engine service ceiling before every flight. Plan your flights so the single-engine service ceiling is well above terrain, ice, and other hazards, so you can still get the single-engine safety a multiengine airplane can provide.
