Avionics Upgrade and Loading Check
An owner of a four-place, single-engine airplane wants to replace an old 12-pound radio at station 20.0 with a 45-pound avionics package at station 60.0. The current empty weight is 1,650 pounds and the empty-weight CG is 38.5 inches aft of the datum. Maximum gross weight is 2,550 pounds. The approved CG range is 35.0 to 47.3 inches aft of datum. Pilot and front passenger stations are 37.0 inches, fuel is at 48.0 inches (40 gallons usable, 6 pounds per gallon), and baggage is at 95.0 inches. The planned flight is two 170- and 150-pound front occupants, full fuel, and 30 pounds of baggage. The mechanic must revise the weight and balance and decide whether the planned load is legal.
1. Compute the new empty weight and empty CG after the equipment change. Is a new weight-and-balance report required?
Read the worked answer
Old empty moment = 1,650 × 38.5 = 63,525 lb-in. Remove: 12 × 20.0 = 240 lb-in. Add: 45 × 60.0 = 2,700 lb-in. New moment = 63,525 - 240 + 2,700 = 65,985 lb-in. New empty weight = 1,650 - 12 + 45 = 1,683 lb. New EWCG = 65,985 / 1,683 = 39.21 in aft datum.
A revised weight-and-balance report is required because empty weight changed by +33 lb and EWCG moved aft about 0.71 in. Also determine whether the installation is a major alteration; if it appreciably affects weight, balance, or structure, it needs approved data, Form 337, and IA or repair-station approval.
2. Compute the planned takeoff weight and CG. Is it within the approved envelope?
Read the worked answer
Pilot: 170 × 37.0 = 6,290 Copilot: 150 × 37.0 = 5,550 Fuel: 40 gal × 6 = 240 lb; 240 × 48.0 = 11,520 Baggage: 30 × 95.0 = 2,850 Added weight = 170 + 150 + 240 + 30 = 590 lb Added moment = 6,290 + 5,550 + 11,520 + 2,850 = 26,210 lb-in Total weight = 1,683 + 590 = 2,273 lb Total moment = 65,985 + 26,210 = 92,195 lb-in CG = 92,195 / 2,273 = 40.56 in
2,273 lb is below 2,550 lb, and 40.56 in is between 35.0 and 47.3 in. The planned load is within the approved envelope.
3. The owner now wants to add two 150-pound rear passengers at station 37.0 and increase baggage to 60 pounds. Determine whether the load is legal; if not, find the minimum fuel to offload.
Read the worked answer
Add two rear occupants: 300 × 37.0 = 11,100 lb-in. Increase baggage from 30 to 60 lb: extra 30 × 95.0 = 2,850 lb-in. New weight = 2,273 + 300 + 30 = 2,603 lb. This is 53 lb over 2,550 lb. New moment = 92,195 + 11,100 + 2,850 = 106,145 lb-in.
Fuel is at station 48.0 and weighs 6 lb/gal. Minimum fuel to offload = 53 / 6 = 8.83 gal, so offload at least 9 gal (54 lb). New weight = 2,603 - 54 = 2,549 lb. New moment = 106,145 - (54 × 48.0) = 106,145 - 2,592 = 103,553 lb-in. CG = 103,553 / 2,549 = 40.63 in, still within limits. Verify actual usable fuel and recompute after loading.