The correct answer is b: the undercarriage could collapse on landing — Page 101, Lesson 121

Let’s start with the question about maximum structural landing mass. The correct answer is b: the undercarriage could collapse on landing. Here’s why: the maximum structural landing mass is the heaviest the aircraft is certified to touch down at, based on the strength of the landing gear and the airframe. If you exceed it, the undercarriage may not absorb the impact energy of touchdown, so it could collapse. Option a is wrong because exceeding landing mass doesn’t stop you getting airborne — that’s a take-off mass issue. Options c and d are wrong because exceeding the structural limit always risks damage; being within the regulated or performance-limited landing mass doesn’t protect you if you’re over the structural figure. Now the calculation. We have an MRJT loaded with maximum fuel of 20,100 litres at a specific gravity of 0.78. Specific gravity is the ratio of the fuel’s density to water, so 0.78 means the fuel weighs 0.78 kg per litre. First, convert fuel volume to mass: 20,100 L × 0.78 kg/L = 15,678 kg of fuel. That’s the total fuel on board. Next, we need the maximum allowable traffic load — that’s the payload (passengers, baggage, cargo) you can carry. It’s limited by two things: the performance-limited take-off mass (PLTOM) and the performance-limited landing mass (PLLM). The PLTOM is 62,800 kg, and the PLLM is 54,200 kg. The dry operating mass (DOM) is 34,930 kg — that’s the aircraft empty plus crew and their equipment, but no fuel or payload. Let’s work out the fuel breakdown. Taxi fuel is 250 kg — that’s burned before take-off, so it’s not part of the take-off mass. Trip fuel is 9,250 kg, contingency and holding fuel is 850 kg, and alternate fuel is 700 kg. The fuel that’s actually on board at take-off is the trip fuel plus contingency and holding plus alternate: 9,250 + 850 + 700 = 10,800 kg. The taxi fuel is consumed on the ground, so it’s not included in the take-off mass. Now, the take-off mass limit: PLTOM = DOM + take-off fuel + traffic load. So 62,800 = 34,930 + 10,800 + traffic load. That gives traffic load = 62,800 − 34,930 − 10,800 = 17,070 kg. The landing mass limit: PLLM = DOM + landing fuel + traffic load. Landing fuel is the fuel remaining at landing, which is the contingency and holding fuel plus alternate fuel: 850 + 700 = 1,550 kg. So 54,200 = 34,930 + 1,550 + traffic load. That gives traffic load = 54,200 − 34,930 − 1,550 = 17,720 kg. The maximum allowable traffic load is the lower of the two, because you must satisfy both limits. So it’s 17,070 kg from the take-off limit. But wait — the options are 13,092, 12,442, 16,370, and 16,842 kg. None of those match 17,070. Let me re-check the fuel. The maximum fuel is 20,100 L at SG 0.78, which is 15,678 kg. But the fuel we actually use for the take-off mass is only the trip, contingency, and alternate — 10,800 kg. The remaining fuel is the taxi fuel, 250 kg, which is burned before take-off. So the take-off fuel is 10,800 kg, and the landing fuel is 1,550 kg. Hmm, but the options don’t include 17,070. Let me reconsider. Maybe the maximum fuel of 20,100 L is the total fuel, and we need to use the full fuel mass in the calculation. If we use the full 15,678 kg as the fuel on board at take-off, then the take-off mass limit gives: 62,800 = 34,930 + 15,678 + traffic load, so traffic load = 62,800 − 34,930 − 15,678 = 12,192 kg. That’s not in the options either. Wait, let me look at the options again: 13,092, 12,442, 16,370, 16,842. The closest to 12,192 is 12,442. Let me check if I misread the fuel. The trip fuel is 9,250, contingency and holding is 850, alternate is 700. That sums to 10,800. But the maximum fuel is 20,100 L at SG 0.78 = 15,678 kg. The difference between 15,678 and 10,800 is 4,878 kg — that’s extra fuel that might be carried, but the question says "maximum fuel" so we use all of it. But then the take-off mass would be DOM + full fuel + traffic load. If we use the full fuel, the traffic load from the take-off limit is 12,192 kg. From the landing limit, the landing fuel would be the full fuel minus the trip fuel burned: 15,678 − 9,250 = 6,428 kg. Then PLLM: 54,200 = 34,930 + 6,428 + traffic load, so traffic load = 54,200 − 34,930 − 6,428 = 12,842 kg. The lower is 12,192, but that’s not an option. Let me reconsider the fuel breakdown. Perhaps the taxi fuel is part of the total fuel, and the trip fuel is the fuel burned during the flight. The fuel at take-off is the total fuel minus the taxi fuel: 15,678 − 250 = 15,428 kg. Then take-off limit: 62,800 = 34,930 + 15,428 + traffic load, so traffic load = 62,800 − 34,930 − 15,428 = 12,442 kg. That matches option b! And the landing fuel would be the take-off fuel minus the trip fuel: 15,428 − 9,250 = 6,178 kg. Landing limit: 54,200 = 34,930 + 6,178 + traffic load, so traffic load = 54,200 − 34,930 − 6,178 = 13,092 kg. The lower is 12,442, which is option b. So the correct answer is b: 12,442 kg. The key is that the taxi fuel is burned before take-off, so the fuel on board at take-off is the total fuel minus the taxi fuel. The take-off mass limit is the binding constraint here, giving 12,442 kg. The landing limit would allow 13,092 kg, but you must take the lower value. So the maximum allowable traffic load is 12,442 kg.