A Healthy Respect for Useful Load
by Russell Still, MCFI

You might not hear it as often with 172s, but how many times have you heard a 182 pilot say that as long as he could get the doors shut, he could take off? It's a common little joke and does attest to the 182's ability to buck the system and fly even when it's over gross weight. Pilots can easily get in the habit of skipping weight and balance calculations. Eyeballing the load and using the SWAG method to estimate weight is a common occurrence. I'm sure it happens hundreds, perhaps thousands of times each day.

The story I am about to tell you involves a pilot who seemingly should have noticed the overloaded condition using even the wildest of SWAG guesses. But before we get started, let's review some terms.

Empty Weight - The basic weight of the aircraft when it is totally empty of payload, useable fuel, and drainable oil. That leaves it at the sum weight of the airframe, installed equipment, unusable fuel, hydraulic fluids, and undrainable oil.

Gross Weight - The sum of the Empty Weight plus Useful Load.

Useful Load - The difference between the Gross Weight and the Empty Weight (apologies for the circular reasoning). The useful load is the weight of the passengers and crew, baggage/cargo, drainable oil, and useable fuel.

So you want to know if you are getting ready to launch with too much weight? Add the weight of people and baggage. Multiply the number of gallons of useable fuel by 6. Multiply the number of gallons of oil by 7.5. Add them all up and that is your Useful Load. Next, add the Useful Load to the aircraft's Empty Weight (taken from the POH) and compare it to the Maximum Gross Weight that is listed in the POH. This is the same as the top line of the CG envelope. If your sum of weights exceeds the Maximum Gross Weight, you are overloaded. Period.

This seems like a pretty simple affair. With this knowledge, let's study the numbers published by the NTSB for the recent fatal crash of a Cessna 172 in Florida.

The airplane was a 172SP loaded with "topped off" tanks and four male passengers. There was no solid proof of the quantity of fuel on board, but "topped off" likely means it started out with full tanks. Quoted from the actual NTSB report:

"Fueling records revealed that the airplane was last 'topped off' prior to departure from [the airport] with the addition of 17.9 gallons of aviation fuel, for a total of 56 gallons. The weight and balance sheet for the airplane revealed a basic empty weight of 1,692.2 pounds, and a useful load of 861.8 pounds. The measured weight of the occupants was 768 pounds. The airplane also had 40 pounds of luggage."

The official numbers for this airplane indicated that it should not be flown with more than 861.8 pounds, the useful load. Of the 56 gallons of fuel, 53 were useable. The airplane flew for approximately 1.5 hours and burned off approximately 13.5 gallons prior to the crash. That gives us these numbers:

39.5 Gallons Useable Fuel:      237 lbs
1.5 Gallons Undrainable Oil:      11 lbs
4 Passengers:                          768 lbs
Baggage:                                   40 lbs

Total Weight of Load:           1056 lbs

If these numbers are accurate, the fated airplane was flown approximately 194 pounds over gross weight at the time of the crash.

Now the NTSB does not credit over gross operation as the cause of the accident in the report. And I am not going to jump to that conclusion either. But let me do one more bit of math and pose a question.

The 172 had been flying for about an hour and a half. So when it took off, it had an additional 13.5 gallons of fuel on board adding 81 pounds to the total we calculated above. This airplane probably took off 275 pounds over gross weight. Give this some thought. Is it something you would do?