Weight and Balance Shift with an Aft Load on a Short Runway
You are planning a departure from a 1 400 m runway at 1 500 ft pressure altitude, OAT +30 °C. The aircraft is a single-engine piston with a maximum take-off mass of 1 150 kg. Basic empty mass is 720 kg; pilot and front passenger 160 kg; rear passengers 150 kg; baggage in the aft compartment 40 kg; fuel 140 L (density 0.72 kg/L). The CG envelope aft limit is 2.45 m from the datum; the aft baggage station is 3.2 m from the datum. The runway has a 1.5% upslope and dry grass surface.
1. Calculate the take-off mass and determine whether it is within limits.
Read the worked answer
Fuel mass = 140 × 0.72 = 100.8 kg.
Total = 720 + 160 + 150 + 40 + 100.8 = 1 170.8 kg.
Maximum take-off mass is 1 150 kg, so the aircraft is 20.8 kg over the limit. This must be resolved before departure - offload baggage or fuel, or reduce passenger load. It is not a marginal rounding issue; it must be corrected.
2. Assess the effect of the aft baggage on CG and whether the load is likely within the aft limit.
Read the worked answer
The aft baggage at 3.2 m is well aft of the 2.45 m limit, so it moves the CG aft. Without full station arms you cannot compute the exact CG, but the qualitative effect is clear: a heavy aft load combined with rear passengers pushes the CG toward or beyond the aft limit. An aft CG reduces static stability and can make the aircraft prone to pitch-up and difficult to recover from a stall.
Action: move or remove the 40 kg aft baggage (and if necessary some rear passenger load) to bring both mass and CG within limits, then recompute. Do not depart with an unverified aft CG.
3. Evaluate the runway and environmental factors and state what performance action is required.
Read the worked answer
At 1 500 ft pressure altitude and +30 °C the density altitude is significantly above field elevation, reducing engine power and lift. Add the 1.5% upslope (increases take-off distance) and dry grass (increases rolling resistance and reduces acceleration). The 1 400 m runway may be inadequate for the actual mass even after correcting the overload.
Action: recompute take-off distance using the aircraft's performance charts for the actual mass, density altitude, slope, and surface. If the required distance exceeds the available runway (with a safety margin), reduce mass further or wait for cooler conditions. Do not assume the book figure for a sea-level paved runway applies here.