When one engine quits on a light twin, the airplane does not become half as capable — it becomes exponentially more demanding to fly. The thrust loss is 50 percent, but the drag caused by asymmetric thrust, a windmilling propeller, and necessary rudder deflection can consume most of the remaining performance. Two speeds cut through that complexity: VYSE, the best single-engine rate-of-climb speed, and VXSE, the best single-engine angle-of-climb speed. Together they answer the two most urgent questions after an engine failure: "How fast am I climbing?" and "How much obstacle clearance am I getting?"
The FAA Airplane Flying Handbook (FAA-H-8083-3C, Chapter 13) treats these speeds as cornerstones of one-engine-inoperative (OEI) training. Every multi-engine pilot must understand not just the numbers but the aerodynamic logic behind them, because the wrong speed choice — even by a few knots — can eliminate what little climb margin exists.
What VYSE and VXSE Mean
VYSE is the calibrated airspeed at which the airplane produces the greatest gain in altitude per unit of time on one engine. It is the single-engine equivalent of VY for an all-engine climb. VYSE is marked on the airspeed indicator as a blue radial line — universally called the "blue line" — making it instantly recognizable without reference to a checklist. Because the marking is required by certification standards, you will find the blue line on virtually every certificated light twin.
VXSE is the calibrated airspeed at which the airplane achieves the greatest gain in altitude per unit of horizontal distance on one engine. It is a steeper, slower climb — useful only when you must clear a specific obstacle before you can accelerate. VXSE is always slower than VYSE, and the margin between them is typically small (often only 5–10 knots). VXSE is not marked on the airspeed indicator; you must know the value from the Pilot's Operating Handbook (POH).
How These Speeds Work: The Aerodynamics
On two engines, climb performance is determined by the excess thrust (or excess power) available above what drag requires for level flight. Lose one engine and two things happen simultaneously: thrust drops roughly 50 percent, and drag increases dramatically — primarily from the windmilling propeller of the failed engine and the large rudder deflection required to counteract the yawing moment created by the remaining thrust. The result is that many light twins have a negative single-engine climb rate at low airspeeds, zero rate somewhere in the middle of the envelope, and a modest positive rate near VYSE.
VYSE sits at the peak of the single-engine power-required versus power-available curve. At that point, the difference between available thrust horsepower and required thrust horsepower is greatest. Flying slower than VYSE increases induced drag faster than it saves anything; flying faster increases parasite drag. Either direction costs you climb rate.
VXSE occupies a slower point on the same curve where the ratio of vertical velocity to horizontal velocity (the climb angle) is maximized. Climb angle depends on the ratio of excess thrust to weight, not on excess power. Therefore VXSE corresponds to the airspeed of minimum drag — the bottom of the total drag curve — which is always slower than VYSE. Because that airspeed is closer to stall and to VMC, operating at VXSE is inherently more hazardous and should be used only when a specific obstacle makes it necessary.
Feathering, Zero Sideslip, and Best Performance
VYSE and VXSE figures in the POH assume the failed engine's propeller has been feathered. A windmilling propeller creates enormous drag — sometimes equivalent to deploying a large spoiler. If the prop is not feathered, actual climb performance will be substantially worse than the published numbers. This is why the memory item "identify, verify, feather" is drilled relentlessly: delaying feathering sacrifices climb performance exactly when you need it most.
Published OEI performance also assumes zero sideslip technique rather than wings-level ball-centered flight. Zero sideslip is achieved by banking approximately 2° into the operating engine while using just enough rudder to eliminate sideslip (the ball will be slightly displaced toward the operating engine, not centered). This position minimizes total drag by aligning the fuselage with the relative wind, reducing the rudder deflection and associated drag. The difference in climb rate between zero-sideslip and wings-level can be 50–150 fpm on a light twin — potentially the margin between climbing and not climbing.
VYSE, VXSE, and Their Relationship to VMC
A critical safety boundary: VXSE must always be above VMC. VMC is the calibrated airspeed below which directional control cannot be maintained when the critical engine (left engine on a conventional twin) fails, with the operating engine at full power and a bank of no more than 5°. If VXSE were below VMC, flying at VXSE during an engine failure could result in loss of control before a climb even begins.
On a conventional light twin, the left engine is the critical engine because its loss leaves the right engine — whose descending blade is farther from the centerline — producing the maximum asymmetric yawing moment. Counter-rotating propeller configurations eliminate the critical engine problem entirely and generally produce more favorable OEI handling.
A related speed is VSSE, the safe single-engine speed: the minimum speed at which intentional OEI training maneuvers may begin. VSSE protects against accidentally inducing actual engine failures or loss of control during practice. VSSE is always at or above VYSE.
Key Numbers and Rules
- VYSE — blue line on the ASI; best single-engine rate of climb; feathered prop and zero-sideslip technique assumed in published data.
- VXSE — not marked on ASI; best single-engine angle of climb; always slower than VYSE; use only to clear a specific obstacle.
- VXSE must exceed VMC — otherwise the published VXSE is operationally unusable.
- Zero sideslip — approximately 2° of bank into the operating engine, ball slightly off center toward the good engine; provides best OEI climb, not wings-level ball-centered.
- Feathering — must be completed promptly; windmilling prop dramatically degrades actual versus published climb performance.
- VMC bank effect — banking 5° into the operating engine can lower VMC by approximately 3 knots per degree (0°–5° range), a fact that is certified and tested at ≤5° bank.
- Temperature and density altitude — VYSE and VXSE are calibrated airspeeds and do not change with altitude, but the actual climb performance achieved at those speeds deteriorates with rising density altitude. Some twins reach a single-engine service ceiling well below the terrain they are flying over.
Why It Matters Operationally
The practical lesson is that VYSE should be established as quickly as possible after an engine failure and feathering confirmation — typically before the gear and flaps are fully retracted on many aircraft. The memory phrase used in training is "Dead foot, dead engine" to identify the failed engine, followed by verify and feather. Once established at VYSE, the pilot has the best possible rate of climb to gain altitude, buy thinking time, and troubleshoot. Descending below VYSE costs climb rate; descending toward VMC risks loss of control.
Reserve VXSE only for genuine obstacle situations — a ridge line, a stand of trees, or a departure end of runway obstruction. The slower speed means less margin above VMC, greater structural angle of attack, and a narrower corridor between safe flight and loss of control. Once clear of the obstacle, accelerate to VYSE immediately.
Single-engine absolute ceiling — the altitude at which VYSE produces zero rate of climb — is a sobering number for most light twins. It is often below 10,000 feet density altitude at gross weight. Planning OEI performance before departure (not after the engine quits) is essential risk management. The FAA's risk management framework (FAA-H-8083-2) would call this identifying a hazard in the planning phase rather than managing a crisis in the air.
Common Test Traps
- Confusing VYSE with VY. VY (all-engine best rate of climb) is a different — usually higher — airspeed. After an engine failure, slowing to VYSE is correct; chasing VY will cost single-engine performance.
- Believing VXSE is marked on the ASI. Only VYSE (blue line) is required to be marked. VXSE comes from the POH and must be memorized.
- Assuming wings-level/ball-centered is optimal. Zero sideslip (2° bank into the good engine, ball slightly displaced) outperforms wings-level in OEI climb. The examiner will ask about this.
- Thinking VXSE is always safe to use. VXSE is above VMC by certification, but the margin may be narrow. At high density altitude and light weight, VMC can actually increase, narrowing that margin further.
- Forgetting that published OEI numbers assume a feathered prop. Students sometimes compute single-engine climb rate from the POH without accounting for how badly a windmilling prop degrades actual performance during the seconds before feathering is complete.