When should I use crossfeed in a multi-engine airplane?

Crossfeed is primarily used after an engine failure, once the failed engine has been secured per the checklist, to allow the operating engine to draw fuel from the inoperative engine's tank and extend range or endurance. It can also be used to correct significant fuel imbalance in normal flight, but only following the specific aircraft's POH procedure. It is not a routine cruise setting.

What is the difference between fuel exhaustion and fuel starvation in a twin-engine airplane?

Fuel exhaustion means the tank is genuinely empty—there is no usable fuel remaining. Fuel starvation means the engine is not receiving fuel that does exist somewhere on the aircraft, typically due to an incorrect fuel selector position or a closed crossfeed valve. Crossfeed mismanagement almost always causes starvation, which is entirely preventable with correct checklist discipline.

How do I know which fuel selector to move when activating crossfeed on a light twin?

This varies by aircraft make and model—there is no universal answer. You must consult the Pilot's Operating Handbook (POH) or Airplane Flight Manual (AFM) for your specific aircraft and study the fuel system schematic before flight. Moving the wrong selector during a single-engine emergency can cut fuel to the operating engine, turning a survivable situation into a fatal one. Know your aircraft's crossfeed procedure cold before you need it.

Multi-Engine Fuel Systems and Crossfeed Operations

Light twin-engine aircraft carry fuel in separate tank systems—one per engine nacelle or wing—that normally feed only their respective engines. This independence is intentional: it prevents a single fuel-system failure from starving both engines simultaneously. However, that same independence creates an operational challenge when fuel loads become unbalanced or when one engine fails. The solution is crossfeed, a plumbing arrangement that allows the operating engine to draw fuel from the opposite tank. Understanding how these systems are designed, when crossfeed is used correctly, and where the traps lie is essential knowledge for every multi-engine pilot.

The FAA Airplane Flying Handbook (FAA-H-8083-3C, Chapter 13) addresses multi-engine fuel system design and crossfeed operations as part of the broader treatment of multi-engine systems and single-engine procedures. The concepts here are directly tested on the multi-engine practical test and appear in checkride oral examinations.

How Multi-Engine Fuel Systems Are Designed

Most light twins use a divided fuel system: the left wing tanks feed the left engine through its own fuel selector, boost pump, and fuel lines; the right wing tanks feed the right engine through a mirror-image system. Each side typically includes main and auxiliary tanks, electric boost pumps (used for takeoff, landing, and high-altitude operation), and a fuel selector that offers positions such as MAIN, AUX, and OFF. Some aircraft also include a crossfeed position on each selector.

The boost pumps serve several functions beyond emergencies. They prime the engine-driven pump at start, prevent vapor lock at altitude, and maintain fuel pressure if the engine-driven pump fails. During normal operations the boost pumps may be left off in cruise to extend their service life, but they must be on for takeoff and landing in virtually all light twins—verify the specific aircraft's POH/AFM for the exact requirement.

Tank Sequencing

When auxiliary tanks are present, the POH will specify the order in which tanks should be used. A common sequence is to take off on mains, switch to aux tanks in cruise to burn them down, then return to mains for landing—ensuring sufficient fuel in the main tanks for go-arounds. Deviating from the manufacturer's sequence can cause unintended fuel imbalance and, in worst cases, fuel exhaustion in one tank while fuel remains in another. Always follow the aircraft-specific POH sequence, not a generic rule.

Crossfeed: What It Is and How It Works

Crossfeed connects the left and right fuel systems so that one engine can draw from the opposite side's tanks. In most light twins this is accomplished by setting the fuel selector of the operating engine to the CROSSFEED position, while the non-operating (or opposite) engine's selector is placed in a position that opens the crossfeed pathway—exact configurations vary widely by aircraft type. The result is a continuous feed path from the opposite tank, through the crossfeed line, to the operating engine's fuel system.

Because crossfeed requires opening valves that are normally kept closed, and because the plumbing varies by aircraft make and model, pilots must be intimately familiar with the specific aircraft's fuel system schematic before using crossfeed. Misidentifying which selector to move is one of the most dangerous errors in multi-engine operations and has contributed to accidents where a running engine was inadvertently starved of fuel.

When Crossfeed Is Used

There are two primary situations that call for crossfeed operations:

Single-engine operations after engine failure: After securing the failed engine and completing emergency procedures, the operating engine may eventually draw down its own tank. Crossfeed allows the operating engine to consume fuel from the inoperative engine's tank, extending range or endurance. This is the classic crossfeed scenario described in training.
Fuel imbalance management in normal flight: If tanks become significantly unbalanced due to fuel system anomalies or improper tank sequencing, crossfeed can be used to feed one engine from the heavier side, correcting the imbalance. However, extended crossfeed operation in normal flight is generally discouraged because it complicates fuel accounting and increases the risk of mismanagement.

Crossfeed is not a routine cruise technique. It is a deliberate, managed procedure used under specific conditions, with the pilot actively monitoring fuel quantities and engine instruments throughout.

Single-Engine Fuel Management: Practical Considerations

When operating on one engine, the surviving engine's fuel demand increases because it is carrying the full performance load. The pilot should:

Complete the engine-failure memory items and the appropriate checklist to secure the failed engine (mixture, throttle, prop to feather, fuel selector off, boost pump off, magnetos off).
Stabilize the aircraft at Vyse (blue-line airspeed) or Vxse as appropriate for obstacle clearance, using zero-sideslip technique—approximately 2° of bank into the operating engine plus rudder—to minimize drag and maximize climb performance.
Monitor the operating engine's fuel supply. If the own-side tank is running low, activate crossfeed per the POH procedure before fuel starvation occurs. Do not wait until a low-fuel warning illuminates.
Track fuel consumed from each tank and maintain awareness of when to switch back to own-side fuel, if applicable.

One critical point: with the failed engine's fuel selector closed (OFF), its tank is isolated. Crossfeed draws that isolated tank's fuel to the good engine. This means the good engine now depends on a tank and lines that may have been exposed to whatever caused the original failure (contamination, fuel exhaustion, or mechanical issue). Before selecting crossfeed, the pilot must consider whether the opposite tank's fuel is actually usable.

Why Crossfeed Matters: Safety and Operational Relevance

Fuel mismanagement remains one of the leading causes of general aviation accidents. In twin-engine operations, the added complexity of crossfeed creates additional opportunities for error. The FAA and NTSB have documented accidents where pilots inadvertently shut off fuel to the operating engine by misusing the crossfeed system, or failed to switch to crossfeed in time and ran the operating engine dry.

The procedural discipline required for crossfeed is directly tied to multi-engine crew resource management principles: verbalize the action, verify the correct selector, and monitor continuously. In a single-pilot twin, there is no copilot to catch errors—checklist use is non-negotiable.

From a performance standpoint, proper fuel management affects weight and balance. As fuel burns from one side, the center of gravity shifts laterally. Most light twins are certified with maximum fuel imbalance limits (expressed in gallons or pounds per side), and exceeding those limits can compromise aircraft controllability. Crossfeed helps maintain balance within limits when needed.

Key Numbers and Rules

Boost pumps ON for takeoff, landing, and engine starts—verify the specific aircraft POH requirement.
Maximum fuel imbalance is specified in the POH/AFM; exceeding it is an operating limitation.
Crossfeed is a managed, checklist-driven procedure—not a casual selector position change.
With the failed engine secured, the fuel selector for that engine should be OFF; the operating engine's selector is then set to CROSSFEED per POH guidance.
Always verify actual fuel quantities with fuel caps and visually (or fuel totalizers) before flight; fuel gauges in light GA aircraft are required to read accurately only at empty—do not trust a gauge reading as a sole source.
Fuel exhaustion vs. fuel starvation: exhaustion means the tank is empty; starvation means the engine is not receiving fuel that exists somewhere onboard. Crossfeed misuse causes starvation, not exhaustion.

Common Test Traps

Confusing crossfeed with fuel transfer: crossfeed allows an engine to burn from the opposite tank in real time; it does not physically transfer fuel from one tank to the other. Some aircraft have a separate transfer system—most light twins do not.
Activating crossfeed without completing the engine-failure checklist first: on the practical test, examiners look for disciplined sequence—secure the engine, then manage fuel. Jumping to crossfeed before feathering the prop or closing the failed engine's fuel selector is a procedural error.
Assuming crossfeed is the same on all twins: selector positions, labeling, and plumbing differ significantly between makes and models. What works in one aircraft may starve an engine in another. Always reference the specific POH.
Neglecting to monitor fuel quantities during crossfeed: students sometimes set crossfeed and forget it. If the opposite tank is inadvertently low or contaminated, the operating engine can fail. Continuous monitoring is required.
Overlooking fuel imbalance limits: a large lateral CG shift from asymmetric fuel burn is an airworthiness issue, not just a performance nuisance. Know the aircraft's limits.