VMC vs VYSE: Red Line, Blue Line & Multi-Engine Meaning
VYSE means the best single-engine rate-of-climb speed. It is commonly identified by the blue radial line on a light multiengine airplane's airspeed indicator. It is a best-performance speed, not a guarantee of a positive climb.
| VMC | VYSE | |
|---|---|---|
| Primary question | Can directional control be maintained? | What speed gives the best one-engine-inoperative rate of climb? |
| Common marking | Red radial line | Blue radial line |
| Concept | Minimum control speed under specified certification conditions | Best single-engine rate-of-climb speed |
| Guarantees a climb? | No | No |
VMC: a certification control-speed reference
VMC is the minimum control speed with the critical engine inoperative under the conditions specified by the applicable certification rules. The published red-line value is therefore a certification datum, not a universal statement that every real engine-out event becomes uncontrollable at one immutable airspeed.
Actual directional-control margin changes with the yawing moment from the operating engine, drag on the inoperative side, rudder effectiveness, CG location, airplane configuration, bank/sideslip, altitude, density, and other factors. This is why multiengine training distinguishes between the published VMC and the actual minimum control speed that could exist in a particular condition.
VYSE: best rate, not guaranteed climb
VYSE is the published best rate-of-climb speed with one engine inoperative. It is represented by the blue radial line on many light multiengine airspeed indicators. At the applicable configuration and conditions, it identifies the speed associated with the best available vertical rate—not a promise that the vertical rate is positive.
If drag exceeds the excess power available from the operating engine, the airplane can descend at VYSE. Weight, density altitude, propeller state, configuration, engine health, and pilot technique all affect the actual result. “Blue line” is therefore a best-performance speed reference, not a terrain-clearance guarantee.
The central distinction: control versus performance
An airplane can be controllable and still be unable to climb. It can also have positive single-engine climb capability while being flown at a speed that provides poor control margin. The FAA's multiengine material treats these as separate problems for a reason:
- VMC: directional control with asymmetric thrust.
- VYSE: best available single-engine rate of climb.
Why actual VMC changes
VMC is driven by a balance of moments. The operating engine creates a yawing moment; the rudder and the aerodynamic effects of bank/sideslip must create an opposing moment. Anything that increases the unfavorable yawing moment or reduces available directional-control authority tends to increase actual VMC. Anything that reduces asymmetric thrust tends to reduce it.
For example, in a normally aspirated piston twin, engine power generally decreases with altitude. Lower power on the operating engine reduces asymmetric thrust, so actual VMC tends to decrease. That does not mean the engine-out situation becomes safer overall: single-engine climb performance is also deteriorating.
Why VYSE and VMC can move in opposite directions conceptually
This is the point worth understanding rather than memorizing. As altitude increases in a normally aspirated airplane, the operating engine may produce less power. Reduced asymmetric thrust can improve the control problem by lowering actual VMC, while the reduced available power simultaneously worsens the performance problem. A lower actual VMC at altitude can coexist with a poorer or negative climb rate.
Zero sideslip and best performance
Best single-engine performance in many conventional light twins requires the manufacturer's recommended zero-sideslip condition, commonly achieved with a small bank toward the operating engine and the appropriate rudder input. The exact bank target and technique are aircraft-specific. Do not substitute a generic bank-angle rule for the POH/AFM or training guidance.
Oral-exam answers
What does VYSE mean? VYSE is the best single-engine rate-of-climb speed.
Which line is blue? VYSE.
Which line is red? VMC.
Does VYSE guarantee a climb? No. It identifies the best available rate; the rate can still be zero or negative.
Is actual VMC always equal to the red-line value? No. The published value is established under specified certification conditions; actual minimum control speed varies with conditions.
Scenario: holding blue line but descending
A training scenario gives you an airplane at its published VYSE with one engine inoperative. Directional control is maintained, but the vertical-speed indication is negative. There is no contradiction: the airplane can be controllable while its best available single-engine rate is a descent.
Separate the questions. The control question concerns the balance between asymmetric yaw and available control authority. The performance question concerns excess power at the actual weight, altitude and configuration. A windmilling propeller, extended gear or higher density altitude can worsen the performance problem without changing the definition of VYSE.
This is a discussion exercise, not an instruction to demonstrate a failure. Use the current POH/AFM chart and approved training procedures to determine the specific airplane’s speeds and capability.
Technical basis
The FAA Airplane Flying Handbook, Chapter 13, explicitly separates the loss-of-directional-control problem from the best-performance problem and associates them with the red radial line (VMC) and blue radial line (VYSE). See the current Airplane Flying Handbook.
Continue the multiengine chain
Back to the full path: Multi-engine checkride study guide.
VMC factors · Critical engine · Zero sideslip · Windmilling vs feathered · Single-engine climb performance