Fuselage, Wings and Stabilizing Surfaces — Page 33, Lesson 43

Let’s start with the structural limitations of the aeroplane. These are the certified maximum masses that the airframe is designed to withstand, and each one applies at a specific phase of operation. First, Maximum Structural Taxi Mass, sometimes called Maximum Ramp Mass. This is the structural limitation of the aeroplane mass at commencement of taxi, meaning at departure from the loading gate. The aeroplane would then burn fuel down to Take-off Mass, or TOM. Next, Maximum Take-off Mass, abbreviated MTOM. This is the maximum permissible mass of the aeroplane including everything and everyone contained in it at the start of the take-off run. Then Maximum Structural Landing Mass, or MSLM. This is the maximum permissible total aeroplane mass on landing in normal circumstances. Now the important one for wing structure: Maximum Zero Fuel Mass, abbreviated MZFM. It is defined as the maximum permissible mass of an aeroplane with no usable fuel. Let me explain why this limit exists, because it’s a structural reason. Bending moments, which apply at the wing root, are maximum when the quantity of fuel in the wings is minimum. During flight, the quantity of fuel located in the wings decreases. As a consequence, it is necessary to limit the weight when there is no fuel in the tanks. That limit value is called Maximum Zero Fuel Mass. So the wing root bending moment is the key — with less fuel in the wings, the wing structure has to carry more of the load, so we cap the weight. Now let’s move to Heavy Landings. Aircraft landing gear is designed to withstand landing at a particular aircraft weight and vertical descent velocity. The maximum vertical descent velocity is 10 ft/sec, or 3.15 m/sec, at maximum landing weight. If either of these parameters — the weight or the descent velocity — is exceeded during a landing, then damage may have been caused to the landing gear or supporting structure, and these loads can be transmitted to the fuselage and mainplanes. Overstressing may also be caused by landing with drift, or landing in an abnormal attitude — for example, nose or tail wheels striking the runway before the main wheels. Some aircraft are fitted with heavy landing indicators, which give a visual indication that specific “g” forces have been exceeded. But in all cases of suspected heavy landings, the flight crew should give details of the aircraft weight, fuel distribution, landing condition, and whether any noises indicative of structural failure were heard. The damage which may be expected following a heavy landing would normally be concentrated around the landing gear, its supporting structure in the wings or fuselage, the wing and tailplane attachments, and the engine mountings. Secondary damage may be found on the fuselage upper and lower skin and structure, depending on the configuration and loading of the aircraft. So to tie it together: the masses are certified limits for taxi, take-off, landing, and zero fuel. The heavy landing criteria give you the design envelope for vertical descent velocity and weight, and the damage pattern tells you where to look after an event. That’s the structural limitation picture for this part of the airframe.