Rear-Mounted Versus Wing-Mounted Engines

There are only two places in an airplane where jet engines are positioned, either in the rear or aft, and at the wings. Both have advantages and downsides, but you seldom see commercial aircraft with rear-mounted or aft-mounted engines anymore. Most commercial aircraft, with the exception of business jets such as the Gulfstream G series and the Cessna Citation, have engines positioned on the wings. In this article, we will contrast rear-mounted or aft-mounted engines with wing-mounted engines.

Only a few commercial aircraft with aft-mounted engines, such as the Embraer ERJ130, ERJ140, and ERJ145, are still in production. Some commercial airlines, such as the Boeing 717, continue to operate rear-mounted engine planes. Many manufacturers, however, no longer make even regional planes with rear-mounted engines; instead, the majority are wing-mounted, such as the Airbus A220, formerly known as the Bombardier CS300.

There are several reasons why most business jets have rear-mounted engines, whereas commercial planes utilize wing-mounted engines. It all relies on how manufacturers construct their planes based on a variety of parameters such as performance, how high they want the plane to sit off the ground, and wing shape. Each has its own pros and limitations, therefore there is truly indeed no “better setup”. The optimal configuration is determined by the aircraft manufacturer.

Wing-mounted engines

Because of the numerous advantages, almost all commercial aircraft manufacturers now put the engines on the wings. First, it makes maintenance easier as they are closer to the ground. Pilots can readily check them as well. Wing-mounted engines also take in clean undisturbed air as the engines are positioned far from any impediments of the fuselage, basically distant from the aircraft structure itself.

Wing-mounted engines can aid in reducing wing bending by adding weight to the structure. It helps decrease the bending pressures on the wing induced by variations in lift force. Because the wings do not need to be as sturdy, this decreases weight and complexity.

However, there are some drawbacks. First, the landing gears must be longer since the engines are lower to the ground and require more ground clearance. As a result, installing larger engines would be more difficult. Case in point are 737s. The vintage 737-200 featured smaller yet longer engines. This enabled the plane to sit low to the ground.

However, due to a shortage of ground clearance, the subsequent 737-300 with bigger engines needed the cowling to have a flat bottom. The 737 MAX, with its bigger CFM LEAP 1B engines, necessitated the engines to be positioned in front of the wing and higher, entirely altering the lift characteristics of the aircraft. This was where the MCAS or Maneuvering Characteristics Augmentation System came in.

Because the engines are located further from the Center of Gravity, planes with wing-mounted engines are more difficult to control in the event of an engine failure. A wing-mounted configuration produces a significant undesirable yaw, which must be corrected by the pilot using the rudder. This might need a larger fin (vertical stabilizer) and rudder design.

Rear-mounted / aft-mounted engines

Planes with aft-mounted engines not only have cleaner wing designs, but they may also sit as low to the ground. This facilitates and expedites freight loading. No need to employ significant freight loading equipment. This was one benefit that the Boeing 737-200 enjoyed during its days. Furthermore, rear-mounted engines necessitate a t-tail design in which the horizontal stabilizers are attached to the vertical stabilizer or fin. I’ll discuss t-tails in another piece, but one advantage is that the elevators are located distant from the center of gravity. This provides more leverage and pitch control, resulting in slower take-off and landing speeds.

However, a t-tail shape leads to profound stall characteristics. Because the engines are usually installed where the horizontal stabilizer would be, a T-tail design may be the only alternative. T-tails are subject to a hazardous condition known as a deep stall. The wake from the wing covers the T-tail in a deep stall, rendering it ineffective. An ineffective tail means the pilots can no longer drive the nose down to recover from the stall.

Compared also to wing-mounted engines, rear-mounted engine arrangements are nearer the aircraft structure such as the fuselage and the back of the main wing. This exposes the engine to wake air from the main wing and disturbed air from the fuselage.

Other disadvantages of aft-mounted engines are:

• A more complex fuel system is necessary. Unlike wing-mounted engines, where the fuel tanks are in extremely close proximity to the engines, aft-mounted engines are farther away and positioned higher. This arrangement requires more powerful fuel pumps and delivery systems. Additionally, if the fuel pumps in the wings fail, the engines might not receive the needed fuel. To ensure redundancy and maintain fuel supply to the engines, stand-by pumps are installed in case the primary pumps fail.
• Increased bending qualities necessitate stronger wings. Without an engine to anchor the wing, aircraft with rear-mounted engines require reinforced wings. This necessitates a stiffer wing structure, which adds weight and introduces other challenges.
• The vertical stabilizer or tail also needs to be stronger. A rear-mounted engine configuration requires a T-tail design, which means the fin must be reinforced to support the horizontal stabilizer adequately.
• In addition to reinforcing the vertical stabilizer, the fuselage must be strengthened to properly support the engines. The area where the engines are mounted is fortified to bear not just the engines’ weight but also the forces generated by variations in engine thrust.

There is no “better” configuration, whether engines are mounted on the wings or at the rear. The choice depends on the manufacturer’s performance goals for the aircraft and the needs of the clients. Both arrangements have their advantages and disadvantages. While commercial passenger aircraft with rear-mounted engines are rare, business jets with wing-mounted engines are similarly uncommon.