Module 2 · Vmc and Single-Engine Performance · Lesson 1 of 4
Lesson 2.1Vmc: Minimum Control Speed
The red line is a number measured under specific conditions. On any given day the real Vmc can be higher or lower.
Rudder effectiveness depends on airspeed. As you slow down with an engine out, you need more and more rudder to hold heading. Eventually the rudder is at full deflection, and any further slowing lets the airplane yaw and roll toward the dead engine no matter what you do. That speed is Vmc.
Vmc is about control only. It says nothing about whether the airplane can climb.
How Vmc is defined
For light twins certificated under the older Part 23 rules (most of the training fleet), 14 CFR 23.149 defines Vmc as the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and then maintain straight flight at the same speed with a bank of no more than 5 degrees. Vmc may not exceed 1.2 Vs1 at maximum takeoff weight.
The 2017 rewrite of Part 23 states it more simply in 23.2135: Vmc is the calibrated airspeed at which, after the sudden loss of critical engine thrust, it is possible to maintain control of the airplane. Airplanes already certificated keep their original basis.
Certification conditions: SMACFUM
- SStandard day conditions
- MMaximum power on the operating engine
- AAft CG
- CCritical engine windmilling
- FFlaps in the takeoff position
- UUp to 5° bank into the operating engine
- MMost unfavorable weight (light weight)
Also: gear up, trimmed for takeoff, out of ground effect. Each SMACFUM condition is set to make Vmc as high as it can reasonably be, so the published red line is close to a worst case.
Recovery limits
- Hold control with no more than 150 pounds of rudder force
- Keep control without reducing power on the operating engine
- Prevent a heading change of more than 20°
- Recover without the airplane taking a dangerous attitude
What moves Vmc
Anything that increases the turning force from the good engine, or reduces the rudder's ability to oppose it, raises Vmc. A lower Vmc is better for control, but some things that lower it also hurt climb performance.
Vmc factor explorer
InteractiveTeaching model of a light twin with a 65-knot red line. The direction of each change matches the FAA Airplane Flying Handbook; the sizes are illustrative. Real Vmc comes from flight testing.
| Change | Vmc | Why | Performance |
|---|---|---|---|
| More power on good engine | Up | More asymmetric thrust | Better |
| Higher density altitude | Down | Normally aspirated engine makes less power, so less yaw | Worse |
| Prop windmilling | Up | Drag on the dead side adds to the yaw | Worse |
| Prop feathered | Down | Removes most of the windmilling drag | Better |
| Aft CG | Up | Shorter arm from CG to rudder, so less rudder authority | Slightly better, less stable |
| Heavier weight | Down | In a bank, more horizontal lift helps hold heading | Worse |
| Less than 5° bank | Up | Up to about 3 knots per degree below 5° | Best near 2 to 3° |
| Flaps extended | Down (usually) | Propwash makes more drag on the operating side | Worse |
| Gear extended | Down (usually) | Keel effect adds directional stability | Worse |
| In ground effect | Up | Less induced drag on the operating side | Better |
| Critical engine fails | Up | PAST effects all work against you | Worse |
Module 2 · Vmc and Single-Engine Performance · Lesson 2 of 4
Lesson 2.2Vmc, Stalls, and Recovery
Losing control and stalling are both dangerous with an engine out. At high density altitude they can arrive together.
Vmc goes down as density altitude goes up, because a normally aspirated engine makes less power and less yaw. Indicated stall speed does not change with altitude. Somewhere above sea level the two lines cross. Above that altitude the airplane can stall before it reaches Vmc.
A stall with full power on one engine and none on the other is the classic setup for a spin. That is why Vmc demonstrations are flown at an altitude where control loss happens before the stall, and why you recover at the first sign of either.
Where Vmc meets the stall
InteractiveTeaching model. Lighter weight lowers stall speed, which moves the crossing point higher. The model holds Vmc fixed as weight changes; in reality lighter weight also raises Vmc a little (Lesson 2.1), so light weight is not purely good. Your instructor may block rudder travel during a demonstration so control is lost at a higher, safer speed.
Recognizing Vmc
- Loss of directional control: full rudder and the nose still turns toward the dead engine
- Stall warning horn or light
- Buffet before the stall
- Controls getting rapidly less effective
Recovering
Do both of these at the same time:
- Reduce power on the operating engineTHROTTLE
Removes the asymmetric thrust that caused the yaw. Idle if needed. - Lower the nosePITCH DOWN
Reduce angle of attack and gain airspeed so the rudder works again.
Once the airplane is under control and above Vsse, bring the good engine's power back up smoothly, adding rudder as you go, and return to straight flight at Vyse. On the checkride, recover within 20° of the entry heading and accelerate to Vxse or Vyse, ±5 knots.
Reducing power to regain control on a real engine failure means descending. Close to the ground, the only real protection is never letting airspeed decay toward Vmc in the first place: rotate no earlier than Vmc + 5 and fly blue line.
Module 2 · Vmc and Single-Engine Performance · Lesson 3 of 4
Lesson 2.3Single-Engine Performance
Climb comes from excess power: what is left after paying for level flight. One engine leaves very little excess.
An airplane climbs when power available exceeds power required to hold altitude. Suppose a twin has 360 horsepower total and needs 160 just to hold altitude at its current weight and density altitude. With both engines, 200 horsepower is left over to climb. Lose an engine and only 180 is available, so the excess is 20 horsepower. Half the power is gone, but 90% of the climb is gone.
Every bit of drag comes straight out of that small excess. A windmilling propeller, extended gear, extended flaps, or flying with sideslip can each turn a slow climb into a descent.
Power available vs. power required
InteractiveTeaching model: 360 hp twin at 3,800 lb. Drag costs are typical light-twin values (windmilling prop and full flaps about 300 to 400 fpm each, gear about 150 fpm). Use your POH for real numbers.
Ceilings
Service ceiling
Highest altitude where the airplane still climbs 100 feet per minute with both engines.
Single-engine service ceiling
Highest altitude where it climbs 50 feet per minute at Vyse, one prop feathered, the other at maximum continuous power.
Single-engine absolute ceiling
Where single-engine climb reaches zero. If you lose an engine above it, the airplane drifts down to it at Vyse.
Climb rate shrinks with altitude
FigureIllustrative light twin at max gross weight on a standard day.
Light twins certificated under the older Part 23 rules (6,000 lb or less, with a low stall speed) are not required to climb at all on one engine. The manufacturer only has to determine and publish the single-engine climb or descent rate at 5,000 feet. On a hot day at gross weight, your twin may not hold altitude on one engine.
Module 2 · Vmc and Single-Engine Performance · Lesson 4 of 4
Lesson 2.4Planning the Takeoff
The decisions you make on the ground decide whether an engine failure after takeoff is survivable.
No regulation stops you from taking off at a weight where the airplane cannot climb on one engine. That makes the planning your job. Section 5 of the POH has charts you did not use in single-engine airplanes. Use all of them, and remember none of them include your reaction time.
Can I stop?
Distance to accelerate to a set speed, have an engine fail, and bring the airplane to a full stop on the runway.
Can I continue?
Distance to accelerate, lose an engine at that speed, feather it, and climb to 50 feet on the other engine.
Can I climb?
Single-engine rate of climb at Vyse for your weight, pressure altitude and temperature.
Will I clear obstacles?
Climb gradient with one engine inoperative, checked against obstacles and departure procedure climb requirements.
Some POHs also give the weight at which you still have a positive single-engine climb at liftoff, and the single-engine service ceiling for a given weight. Taking off at the lowest of these weights carries the least risk. Flying above it is legal, but you accept that a failure at certain points will force a landing.
Engine failure on takeoff: what is the plan?
InteractiveThe zones match a typical pre-takeoff briefing. Brief them out loud before every takeoff so the decision is already made.
Runway check: accelerate-stop and accelerate-go
InteractiveExample distances for a light twin at gross weight, growing with density altitude. Real numbers come from your POH charts.
Clearing obstacles on one engine
A climb gradient is height gained per horizontal distance. At 2.5%, you climb 2.5 feet for every 100 feet you travel. Measure from the point of liftoff to the obstacle, not from the start of the takeoff roll.
One-engine climb gradient calculator
InteractiveFeet per minute = feet per NM × groundspeed ÷ 60. A gradient in percent × 60.76 gives feet per NM. Compare the result with the single-engine climb chart.
Module 2 · Vmc and Single-Engine Performance · Knowledge Check
Knowledge CheckModule 2 Knowledge Check
Ten questions an examiner could ask about this module. Each answer comes with the reason. Pass mark: 80% (8 of 10).