Twin Proficiency: They Both Need Fuel

Twin Proficiency: They Both Need Fuel

Twin Proficiency: They Both Need Fuel

In most twin-engine airplanes, fuel management is simple. Each engine’s fuel selector often has three positions: ON, OFF and CROSSFEED. CROSSFEED is an emergency position, for use during extended single-engine operation to maximize range and balance fuel when one engine is shut down. Consequently, the fuel selector typically remains in the ON position.
Some airplanes have auxiliary fuel tanks, resulting in a fourth valve position: AUX. In every airplane I’ve encountered, independently-selectable auxiliary fuel tanks are POH-limited to cruise flight. That adds only a little complexity to the task of fuel management: take off with the selector in ON or MAIN, switch to AUX at some point during cruise, and return to ON before descent and landing.
Regardless of your airplane’s fuel system specifics, one thing seems obvious: both engines need fuel in order to run. Since you generally start one engine right after the other, the engines usually burn fuel at approximately the same rate, and we usually load the same amount of fuel into each wing, for symmetry. One engine starving for fuel is way down on our list of things to worry about.
Yet, it happens. What are some of the unusual scenarios that result in exhausting one engine’s fuel supply, while fuel is available to the other? What strategies are suggested by studying these events?
From the NTSB:
The pilot of a Beechcraft Baron E55 noted an imbalance between the left and right main fuel tanks during cruise. He attempted to correct the imbalance by placing the left fuel selector in the crossfeed position, so both engines would receive fuel from the right main tank. About 15 minutes later, both engines lost power. When the pilot reset the left and right fuel selectors to the left main and the right auxiliary fuel tanks, respectively, the left engine regained power and the right engine began “surging.”
The pilot decided not to shut down the right engine and diverted to the nearest suitable airport. During final approach, the right engine lost power completely, and the airspeed decayed until it approached the airplane’s minimum-control airspeed. When the airplane drifted right of the runway centerline, the pilot reduced power on the left engine in an attempt to maintain control. The airplane impacted an open field near the runway and a post-impact fire ensued.
Post-accident examination revealed no airframe or engine anomalies consistent with a pre-impact mechanical failure or malfunction. Both fuel selectors were positioned to their respective main fuel tanks. The pilot stated that the airplane was fully fueled before departure. He stated he had not used any of the fuel in the auxiliary tanks before the loss of engine power. The pilot’s operating handbook notes that the crossfeed system is not to be used to transfer fuel from one tank to another or to balance fuel during flight. The simultaneous loss of engine power while both engines were receiving fuel from one fuel tank, the restoration of power on the left engine after switching fuel tanks, operation at or near the endurance limit for the right main tank, and the lack of anomalies identified during the engine exams are consistent with exhaustion of fuel in the right main tank.
NTSB probable cause: The pilot’s failure to maintain airspeed and subsequent loss of airplane control during a single-engine landing approach. Contributing to the accident was the pilot’s improper fuel management during cruise flight, resulting in an initial loss of power in both engines and the subsequent single-engine approach after power was restored on only one engine.
From the NTSB:
Toward the end of a 6 hour, 20 minute flight, during a night visual approach, the pilot of a Cessna Conquest flew the airplane to a left traffic pattern downwind leg, lowering the landing gear and setting the flaps to 30 degrees. He turned to a left base leg and, after doing so, was heard on the CTAF stating that he had an “engine out.”
The airplane then passed through the final leg course, the pilot called “base to final,” and the airplane commenced a right turn while maintaining altitude. The angle of bank was then observed to increase until the wings became vertical, then inverted, and the airplane rolled into a near-vertical descent, hitting the ground upright in a right spin.
Subsequent examination of the airplane and engines revealed that the right engine was not powered at impact, and the propeller from that engine was not in feather. No mechanical anomalies could be found with the engine that could have resulted in its failure. The right fuel tank was breeched; however, fuel calculations, confirmed by some fuel found in the right fuel tank as well as fuel found in the engine fuel filter housing, indicated that fuel exhaustion did not occur. It is unknown if or how much pilot fatigue might have influenced the outcome.
NTSB probable cause: The pilot’s failure to maintain minimum control airspeed after a loss of right engine power, which resulted in an uncontrollable roll into an inadvertent stall/spin. Contributing to the accident was the failure of the airplane’s right engine for reasons that could not be determined because no preexisting mechanical anomalies were found, and the pilot’s subsequent turn toward that inoperative engine while maintaining altitude.
From the NTSB:
The Cessna 402B was approaching the destination airport with about seven gallons of fuel remaining in each of the 50-gallon main fuel tanks. While on short final, as the pilot fully extended the flaps, the airplane experienced a total loss of power on the right engine. The airplane touched down hard on a grassy area about 650 feet before the runway, bounced onto the runway, and came to rest. The pilot subsequently started both engines and taxied to the ramp area uneventfully.
A post-accident examination of the airplane revealed that the right main fuel tank’s fuel transfer pump was inoperative. The fuel transfer pump was designed to operate continuously when the battery switch is on. The purpose of the pump is to transfer fuel from the nose section of the tank to the fuel pick-up area near the center of the tank, which permits steep descents with low fuel quantity. It could not be determined when the fuel transfer pump failed; however, confirming pump operation is a required preflight inspection item for each main fuel tank.
NTSB probable cause: The failure of the right main fuel tank transfer pump, which resulted in a total loss of right engine power during approach due to fuel starvation, and a subsequent hard landing. Contributing to the incident was the pilot’s operation of the airplane with a low fuel quantity.
Let’s look at some commonalities in these three scenarios, and explore some strategies to avoid finding yourself in similar circumstances.
Common Elements
Low fuel level. It sounds redundant to say that fuel exhaus-tion was due in part because the fuel level was low, but work with me: in these three events, the pilot was knowingly operating the aircraft to the extreme of its fuel endurance. In the case of the Conquest and 402B, it appears the pilot intended to stretch the airplane’s range to its limits. In the Baron, some unusual situation led to detection of a fuel imbalance, and the pilot flew the airplane into a limited-fuel condition.
Lessons from this: Don’t assume that mathematics, and even your knowledge of the fuel system, will precisely predict results in actual flight. Concepts of unusable fuel and regulatory fuel reserves are designed to give us a little “wiggle room” if actual conditions differ from what we expect. If you’re operating near the maximum range of the aircraft (“maximum” may be revised if something unusual happens en route), be spring-loaded to divert to a nearby airport if you cannot positively verify the amount of fuel remaining through multiple, independent means.
Airplane limitations. Airframe and systems limitations are almost always the result of known and documented issues that stem from the aircraft’s design–often accentuated by examples in the NTSB record.
The Baron pilot apparently adhered to the fuel system limitation, at the last minute, because both selectors were found on their respective main tank. Perhaps he switched before the engine quit and exhausted the remaining fuel in that tank. It’s possible the engine failed while burning from the auxiliary tank, and he switched to the main tank as part of a restart attempt.
In the Conquest, the right engine did not completely empty its tank, but the fuel level was apparently pretty low. It may be that a common fuel system limitation was a factor–to avoid slips when the fuel level is below some level. More correctly, this condition is problematic when making an uncoordinated turn, when a low fuel level can “slosh” away from the fuel lines.
The Cessna 402B’s case should have been obvious before takeoff. An inoperative fuel transfer pump is a “no-go” item in these airplanes. If the pilot performed the pre-takeoff check, and assuming the pump failure did not occur at some time during the accident flight, he should have aborted the flight. Good fuel system knowledge would tell him there’s a real reason to avoid departing with an inoperative transfer pump, especially if flying to the maximum endurance of the airplane.
Loss of control on one engine. Regardless of why an engine quits, it’s vital to apply proper control inputs to establish and maintain control. Much multiengine training focuses on controlling the airplane through the initial stages of an engine failure, including feathering the correct propeller. Less time and training effort, typically, is spent on single-engine approach and landing. A few trips around the pattern with an instructor, with one engine in zero-thrust from downwind to landing, simulating engine-out configuration, can remind you of what’s important after an engine quits.
Fuel leaks, blocked fuel vents, loose fuel caps, unexpected variations in fuel burn, and asymmetric fuel loading can cause one engine to run out before the other…unexpectedly. Attempting extreme-range flight, with uncoordinated flight during turns in the pattern, can unport a fuel tank and cause one engine to quit. Mechanical failures with fuel transfer and delivery systems can make an engine fail suddenly.
It’s possible to have fuel exhaustion on one engine while fuel is still available to the other. Watch for the unusual situations when this may occur, and land early if you suspect it may be happening to you.

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