It’s a question many passengers ponder as they settle into their seats: can an airplane fly with just one engine? While it might seem like a precarious situation, modern commercial airplanes are designed with engine redundancy in mind. Let’s delve into the factors at play when considering single-engine flight capability.
Initially, a simple comparison of thrust and weight might suggest that a single engine could suffice. Looking at examples like the Boeing 747-400 (B744) and the Airbus A380-800 (A388), we can see some interesting figures. The B744, with a Maximum Take-Off Weight (MTOW) of 4050 kN, requires approximately 225 kN of thrust to maintain altitude, assuming a Lift-to-Drag ratio (L/D) of 18. Its static thrust, however, ranges from 276–282 kN, seemingly providing a comfortable margin. Similarly, the A388, boasting a MTOW of 5640 kN, needs around 313 kN to stay aloft, while its static thrust is between 332–356 kN. These numbers, at first glance, might lead one to believe that a single engine is more than capable.
However, the crucial aspect often overlooked is that jet engine thrust isn’t constant; it fluctuates significantly with both speed and altitude. At lower altitudes, engine thrust remains relatively stable until it reaches its peak pressure limit, after which it begins to decrease with air density changes. Speed also plays a role. Initially, thrust decreases as speed increases, but as the aircraft gains speed and takes advantage of ram pressure recovery at higher Mach numbers, thrust can actually increase again.
At low altitudes, where engines still produce near their rated thrust, the optimal L/D speed is typically around Mach 0.3–0.35, just above the point of minimum thrust. It’s estimated that thrust reduction at this speed can be about 20%. Applying this reduction:
- For the B744 with the most powerful engine, 282 kN × 0.8 = 225.6 kN. This is just barely sufficient to maintain altitude under ideal conditions.
- For the A388, 356 kN × 0.8 = 284.8 kN. This is notably less than the required 313 kN, indicating a single engine would not be enough in this scenario.
Furthermore, these calculations are based on an idealized L/D ratio. In reality, flying with asymmetric thrust from a single engine necessitates rudder and aileron deflection to maintain control, which increases drag and worsens the L/D ratio. Therefore, relying on a single engine at low altitude, even for an aircraft like the B744, is far from ideal and leaves very little margin for error.
At cruising altitudes, typically around Mach 0.7–0.75, engines operate at speeds where thrust is theoretically higher than static thrust due to ram pressure. However, at these altitudes, engines only produce about a quarter of their sea-level thrust due to the thinner air. This drastic reduction means that all engines are essential for maintaining flight at cruise altitude. In fact, cruise altitude selection is always just below the aircraft’s operating ceiling – the altitude where the aircraft can no longer sustain a climb rate of more than 500 feet per minute with all engines operating. If an engine is lost at cruise altitude, an immediate descent is always necessary to maintain airspeed and prevent stalling.
In conclusion, while calculations might suggest marginal single-engine capability under very specific and ideal low-altitude conditions for certain aircraft like the B744, it’s generally insufficient, especially for larger aircraft or at higher altitudes. Modern commercial airplanes are designed to safely fly and land with one engine inoperative, which is a critical safety feature. However, this is a contingency measure to reach the nearest suitable airport, not a standard operating procedure. The redundancy of multiple engines is crucial for maintaining efficient and safe flight across a wide range of conditions, especially at cruise altitude where performance demands are highest.
