reSource Course · Multi-Engine Add-On · Module 2

Vmc and Single-Engine Performance

How slow you can go and still keep control, how much climb is left, and how to plan a takeoff around both.

Four lessons cover Vmc and the SMACFUM certification conditions, how Vmc and the stall interact, why one engine leaves so little climb, and the takeoff planning that decides whether an engine failure after takeoff is survivable.

Start Lesson 2.1 Test Yourself

4Lessons

7Interactive labs

10Check questions

Which way does Vmc move?

Interactive

Module 2

Lessons

4 lessons, one page each. Work through them in order and mark each one complete, then test yourself with the knowledge check.

Assessment

?Module 2 Knowledge CheckTen questions an examiner could ask about this module. Each answer comes with the reason. Pass mark: 80% (8 of 10).

Module 2 · Vmc and Single-Engine Performance · Lesson 1 of 4

Lesson 2.1

Vmc: 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.

Determined with

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.

At Vmc the pilot must be able to

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

Interactive

Power on operating engine 100%

Density altitude 0 ft

Bank toward operating engine 5°

Dead engine prop

Center of gravity

Weight

Flaps

Landing gear

Height

Engine failed

Actual Vmc65 kt

vs. red line0 kt

Climb performance—

SMACFUM conditionsThese are the conditions used to set the published red line, so actual Vmc matches it. Change one to see which way Vmc moves.

Teaching 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 2.2

Vmc, 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

Interactive

Density altitude 3,000 ft

Weight Max gross

Vmc here—

Stall here—

Lines cross at—

Teaching 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 engine
Removes the asymmetric thrust that caused the yaw. Idle if needed.
THROTTLE
Lower the nose
Reduce angle of attack and gain airspeed so the rudder works again.
PITCH DOWN

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.

In a real failure

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 2.3

Single-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

Interactive

Engines

Power to hold altitude (weight, density altitude) 160 hp

Drag on the dead side and airframe

Excess power—

Rate of climb—

Climb lost—

Teaching 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

100 fpm

Service ceiling

Highest altitude where the airplane still climbs 100 feet per minute with both engines.

50 fpm

Single-engine service ceiling

Highest altitude where it climbs 50 feet per minute at Vyse, one prop feathered, the other at maximum continuous power.

0 fpm

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

Figure

Illustrative light twin at max gross weight on a standard day.

No guarantee

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 2.4

Planning 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.

Accelerate-stop

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.

Accelerate-go

Can I continue?

Distance to accelerate, lose an engine at that speed, feather it, and climb to 50 feet on the other engine.

Climb, one engine

Can I climb?

Single-engine rate of climb at Vyse for your weight, pressure altitude and temperature.

OEI gradient

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?

Interactive

Move the airplane to the moment the engine fails

The 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

Interactive

Runway available 4,000 ft

Density altitude 2,000 ft

Example 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

Interactive

OEI gradient (%)

Takeoff roll (ft)

Obstacle distance from start of roll (ft)

Obstacle height (ft)

Departure procedure requires (ft per NM)

Groundspeed (knots)

Feet 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 Check

Module 2 Knowledge Check

Ten questions an examiner could ask about this module. Each answer comes with the reason. Pass mark: 80% (8 of 10).

Score0 / 10

Next: Module 3 · Procedures, Systems, and the Checkride