Why the 737 MAX Did Not Lengthen the Landing Gear: Design Choices and Implications

The Boeing 737 MAX, known for its innovative cockpit and efficient CFM International LEAP engines, involves several design decisions and trade-offs that were necessary for its certification and operational advantages. One such decision was the rejection of lengthening the landing gear struts to increase ground clearance for the engines. This choice had to balance the practical considerations of ground clearance, pilot certification, and aircraft design.

Ground Clearance and Aesthetic Design

The main landing gear struts on the 737 MAX8 are 6 inches longer than those on the 737–800. This 6-inch increase is the maximum possible length without causing the tires of the left and right landing gear to touch when retracted. Any further increase would require redesigning the entire landing gear attachment points, likely necessitating a significant redesign of the aircraft to achieve such an increase. This redesign process is complex and would significantly alter the aircraft's type certification, making the MAX a different type from previous 737 models.

Key Considerations:

Design Limitation: The hinges can only be placed so far apart while remaining attached to the fuselage, limiting the lengthening of the landing gear struts. Alternative Redesigns: Significant changes would involve moving the hinges further apart, requiring a new landing gear attachment point, possibly external fairings, or more complex folding mechanisms. Such changes would necessitate a new type certificate and would not be backward compatible with existing pilots or aircraft types.

Influence on Pilot Certification and Training

One of the appealing aspects of the 737 MAX was the ease of transition for pilots. A pilot who is certified on the 737–600, 700, 800, 900, -300, or -400 can complete a day course for differences training and fly the MAX that afternoon. This was made possible by maintaining the aircraft's overall design and trajectory as closely as possible to the original 737 models, ensuring a training gradient from the older models to the MAX.

The full design of the 737 MAX was intended to be similar to existing 737s, allowing for pilots to maintain their skills and qualifications with minimal disruption. The whole aerodynamic design of the aircraft needed to remain consistent to this extent, with the landing gear height being a critical factor in maintaining this design consistency.

Aerodynamic Impact: Nose Height and Engine Placement

The 737 MAX also features a 6-inch taller nose wheel due to the aircraft's ground attitude being designed to be relatively flat. An excessively tall nose wheel could cause the plane to sit in a nose-high position, leading to various operational challenges, including difficulty in loading and unloading, issues with taxiing, and potential for nose-first touchdowns. These characteristics are important for pilot safety and ground operations.

Additional Factors:

Forced High Angles: Higher nose positions during ground operations can make it challenging for pilots to accurately judge distances, leading to nose-first touchdowns which could result in structural damage. Aerodynamic Considerations: The engines, placed further forward due to their size, contribute to the aircraft's nose-heavy condition, which can be exacerbated by the flat lifting surfaces beneath the engines.

Redesign Challenges and Integrity

The 737 MAX's ability to generate lift beneath its engines, much like an accidental canard wing, necessitates careful engineering. Under normal cruise conditions, this effect is minimal, but at higher angles of attack, the lift distribution can cause a pitch-up effect. This is particularly relevant for maintaining stable flight characteristics.

Bosch proposed several additional tweaks, including using a stick pusher (akin to the 727, DC-9, Q400, and ERJ145) as a simple solution. However, Adopting such a system would have required significant changes to the existing 737 design and would have necessitated a new type certificate, further complicating the transition for pilots.

Summary: The choice to keep the height of the landing gear and nose significantly influenced the overall design and regulatory environment of the 737 MAX. It ensured a smooth transition for pilots and a consistent aircraft type, avoiding the complexities and costs associated with a major redesign.

References:

Design Limitations of Landing Gear Struts Pilot Transition and Certification for 737 MAX Aerodynamic Impact of Engine Placement and Nose Height

Design Limitations of Landing Gear Struts

Hinges and Infrastructure Restrictions Impact on Pilot Transition and Ground Operations Redesign and Regulatory Costs

Pilot Transition and Certification for 737 MAX

Ease of Transition for Pilots Certified on 737-300 to 737-900 Day Course for Differences Training Consistency with Existing 737 Models

Aerodynamic Impact of Engine Placement and Nose Height

Flat Ground Attitude and Nose Position Lift Distribution and Pitch-up Effect Industry Examples of Stick Pushers