Can the Airbus A380 safely fly with two engines out on the same wing?

Question

I know twin-engine jets can safely take off and fly with one engine out (i.e. all thrust only on one side). Of course quad-engine jets are also able to fly safely in case one engine is out.

But is the A380 designed to also fly safely in case two engines on the same wing are out?

Would the rudder be able to compensate the thrust imbalance?

I'm referring to an engine-out situation in flight, not during take off. I would assume that the problems aggravate during take off since the rudder has less airflow to counter the imbalance.

Note: Two engines out on the same wing is not a very likely scenario, but it's not impossible. For example, on Qantas 32, damage from the number 2 engine explosion impacted fuel and control lines of engine 1.

Answer 1

Proving that a multi-engine aircraft is flyable with one or more engines inoperative is part of the certification programme. The Airbus A380 is certified under FAR (Federal Aviation Regulations, USA) part 25 and JAR (Joint Aviation Regulations, Europe) part 25. One of the requirements of FAR/JAR 25 is that the directional control can be maintained when two critical engines (that means under the same wing) fail.

The following is selectively extracted from FAR part 25 (JAR is similar, there are some differences). Emphasis added by me.

Controllability and Maneuverability

§25.143 General.

....

b) It must be possible to make a smooth transition from one flight condition to any other flight condition without exceptional piloting skill, alertness, or strength, and without danger of exceeding the airplane limit-load factor under any probable operating conditions, including—

1. The sudden failure of the critical engine;

2. For airplanes with three or more engines, the sudden failure of the second critical engine when the airplane is in the en route, approach, or landing configuration and is trimmed with the critical engine inoperative;

...

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§25.147 Directional and lateral control.

a) Directional control; general. It must be possible, with the wings level, to yaw into the operative engine and to safely make a reasonably sudden change in heading of up to 15 degrees in the direction of the critical inoperative engine. This must be shown at 1.3 V_SR1 for heading changes up to 15 degrees (except that the heading change at which the rudder pedal force is 150 pounds need not be exceeded), and with—

1. The critical engine inoperative and its propeller in the minimum drag position;

2. The power required for level flight at 1.3 V_SR1, but not more than maximum continuous power;

3. The most unfavorable center of gravity;

4. Landing gear retracted;

5. Flaps in the approach position; and

6. Maximum landing weight.

b) Directional control; airplanes with four or more engines. Airplanes with four or more engines must meet the requirements of paragraph (a) of this section except that—

1. The two critical engines must be inoperative with their propellers (if applicable) in the minimum drag position;

2. [Reserved]

3. The flaps must be in the most favorable climb position.

c) ...

...

d) ...

e) Lateral control; airplanes with four or more engines. Airplanes with four or more engines must be able to make 20° banked turns, with and against the inoperative engines, from steady flight at a speed equal to 1.3 VSR1, with maximum continuous power, and with the airplane in the configuration prescribed by paragraph (b) of this section.

f) ...

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§25.149 Minimum control speed.

...

g) For airplanes with three or more engines, V_MCL-2, the minimum control speed during approach and landing with one critical engine inoperative, is the calibrated airspeed at which, when a second critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with both engines still inoperative, and maintain straight flight with an angle of bank of not more than 5 degrees. V_MCL-2 must be established with—

1. ...

   ...

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§25.161 Trim.

...

e) Airplanes with four or more engines. Each airplane with four or more engines must also maintain trim in rectilinear flight with the most unfavorable center of gravity and at the climb speed, configuration, and power required by §25.123(a) for the purpose of establishing the en route flight paths with two engines inoperative.

Answer 2

Would the rudder be able to compensate the thrust imbalance?

It depends.

The best way to translate those regulations into practical examples is to look at the certification of the A330 and A340-300, which have approximately the same amount of total thrust, the difference being the engine configuration.

Takeoff performance requirements are predicated on 1-engine out for both configurations. Hence you'll notice that the minimum climb gradient requirement means that the 4-engine loses less thrust, hence is capable of a higher takeoff weight; around 30-40T more.

Two engine out comes into play during the landing config, where VMCL2, two engine out minimum control speed in the landing config, is much higher than VMCL1; two engine out 340 has approximately the same amount of thrust as a one engine out 330, but the thrust line produces a greater moment on the 340. Incidentally these speeds don't change with weight and depend on handling qualities and directional control at the most unfavorable CG position, which is the AFT limit. VMCL2 is around 160 knots, which is well above normal landing speed, so at some point during the approach when landing is assured, the pilot will elect to reduce below the VMCL2 to normal landing speed. It will not be possible to select full thrust without losing directional control in this regime of flight, however the benefits of landing at lower speed offset this.

Back to the takeoff case with two engines inoperative, although VMCL is not specifically defined for takeoff config and regulations don't specifically cover this scenario, at higher takeoff weights minimum control speed becomes less of an issue compared to VS1G and overall climb performance. While at lighter weights, it maybe possible to reduce thrust to maintain control if performance permits.