Accuracy in calculations

• Posts: 2261

bobtait created the topic: Accuracy in calculations

Since CASA introduced the 'type-in' answers for questions involving calculations, some students have been losing sleep over the second decimal place of a litre or a 5 knot discrepancy in wind component on a P chart. I must admit that, without some indication from CASA of the margins applied to the acceptable answers, I can understand the frustration.

Most of the answers given in the books were calculated on the back of a navigation computer and read off the scale provided. That includes PNR and ETP calculations. I haven't seen any warning that this method is no longer considered valid but it does seem that a student would be wise to use a calculator for all such questions and keep all intermediate decimal places before rounding the final answer up or down as required.

I turned 80 this week, so I consider I'm overdue to get a few things off my chest. I grew up in the days when a little common sense prevailed. I must admit that, by today's standards, we would be considered a bunch of reckless cowboys who threw caution to the wind an calculated fuel to THE NEAREST GALLON! Oh my God, how on earth did we ever survive?

We understood that If you are going to estimate the actual fuel on board by poking a stick into the fuel tank and checking to see how much of it gets wet, it really is rather ridiculous to then do the remaining calculations to a hundredth of a litre. Also, we make an arbitrary allowance for the fuel used during taxi, which almost certainly is not exactly accurate.

To continue with this nonsense, we dip the tanks and get an answer in litres or gallons (that's a measure of volume) when the energy contained in the fuel depends upon its mass - not its volume. Maybe we should introduce a further step in the calculation to allow for specific gravity and temperature. If you don't put a spirit level on the wings, the dip will be wrong because the fuel may not be evenly distributed within the tank.

We also understood that most runways have a varying slope, and wind components estimated by observing a wind soc 100m away is not likely to be perfectly accurate. Most importantly, we understood that P Charts have very generous margins to allow for all of this and the whole idea is to provide the pilot with an assurance that take-off or landing at a particular strip under given conditions with be well and truly safe.

Don't even start me on centre of gravity calculations using arms to a fraction of a mm when the cargo in a compartment is not likely to be distributed symmetrically over the exact geometric centre of the compartment floor.

OK. I feel better now........
The following user(s) said Thank You: alibkord

• John.Heddles
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• ATPL/consulting aero engineer
• Posts: 670

John.Heddles replied the topic: Accuracy in calculations

Bob,

Well done, good sir, on passing the 80-candle-blowing-out-rite-of-passage!

Thoroughly concur with your accuracy philosophy.

My very best regards for the birthday,

John

Engineering specialist in aircraft performance and weight control.

• Posts: 2261

bobtait replied the topic: Accuracy in calculations

Thanks John. It's always a pleasure to hear from you. Your contributions are valuable and much appreciated.

Bob