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Plus, like most aspects of our lives, computers, tablets, smartphones and, for pilots, avionics have relieved us from tedious (often less than per- fect) manual math calculations. And this is a good thing not only because of the three types of pilots above but also because four out of three pilots are dyslectic.
WAG’s and TLAR
I tell student aviators that if they can add, subtract, multiply and divide, then that will do just fine. A lot of our math is the “cut it with an ax, measure it with a micrometer” type calculations anyway – meaning that it’s mostly used for estimation and our situational awareness. It’s a WAG (Wild *ss Guess) with a quick “reasonable/reality” check to verify TLAR (That Looks About Right). If the calculations are so critical that we need an answer down to the single mile, to one minute, one gallon, one pound or to several significant digits, then we are probably cutting it way too close – or taking an FAA exam.
Most of our math calculations are used for a time, distance and fuel consumption analysis. The distance we travel in one minute times 60 gives us our GS. We can then divide the distance to our fix/destination by that one-minute calculation to get time remaining to the fix. We multiply the time to the fix by the gph or pph we are burning (with a minor conversion of minutes to hours), and we have fuel used to the fix. We subtract the fuel to the fix from the fuel remaining, and we get fuel remaining at the fix.
All this, plus-or-minus the head- wind/tailwind, and we arrive at a TLAR WAG – all accomplished in our head without an Etch A Sketch or E6-B. And speaking of wind, one thing that I find that even experi- enced pilots often don’t wrap their head around is that once airborne – except for abrupt and significant changes in velocity, direction, den- sity or temperature – the airplane doesn’t know about, or care about, wind. From the airplane’s anthro- pomorphic perspective, it’s only us math-challenged pilots that need such esoteric information as wind
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speed and direction relative to the ground. We do all of this (sans E6-B) in order to learn ETA’s, fuel require- ments and then crosswind and land- ing distances when physical contact with the earth is imminent.
If you were to display an E6-B to the passengers or crew when mak- ing math calculations, you might get worried stares regarding your competence – kind of like the Chair of the Federal Reserve counting on his fingers during a press confer- ence. Modern avionics, f light plan- ning and weight and balance apps/ programs have made pref light and inf light math unnecessary. Airframe and powerplant limitations, V-speed and navigation figures can now be presented on multifunction displays or the engine and f light instruments themselves. Computer-based f light planning programs are common and can be accessed enroute via the internet, Airtext or SATCOM. Enroute climb and descent points, RNAV climb-via and descend-via ar- rivals and departures, ETA’s, holding patterns and speeds are all calcu- lated by the CADC, GPS, AHARS and FMS. Are there applications of old-school math for pilots that we should remember? You bet – because you never know when some or all of our electronic magic will disappear. Even when our electronics work, most of us still use mental math as backup confirmation for one par- ticular calculation.
Pilot Arithmetic: A Practical Application
The TOD (Top of Descent) cal- culation is an example of usable old-school math. The easiest way to compute TOD is to use the VSI and a GPS that shows time and distance to the fix. If you want to be over fix ABC at 10,000 and you are at 30,000 now, you have 20,000 to lose. If you begin your descent at 20 minutes to go, that’s 1,000 fpm. If you wait until 10 minutes to go, that’s 2,000 fpm. Of- ten, ATC likes to give us a clearance that may be something like, “Cross 65 miles Southeast of ABC at FL190 and 280 knots, expect to cross ABC at 10,000.” Of course, the easiest way is to enter this in your FMS.
Excluding that, and in order to exercise our math brain cells, sub- tract the altitude at which you want to be (19,000) from the altitude at which you are (30,000). That’s 11,000. Multiply by 3 and drop the zeros (11,000 x 3 = 33 miles) and add that to the fix distance (65 + 33 = 98 miles) and start down at 98 miles. Recalculate every couple thousand feet and adjust your vertical speed to compensate for changing wind. Do these calculations before you enter it into the FMS so that you may be- gin the descent immediately if the clearance was given late i.e., a slam dunk. Then, the next descent from 19,000 to 10,000 would be: (19-10) x 3 = 18. Start the descent at 18 miles from the fix. Ah, mathematics – how refreshing when we find a practi- cal application.
Use Your Fingers
Don’t let old-school math scare the derivatives out of you. When working those pilot-y story problems, don’t get distracted by irrelevant compo- nents like pizza, watermelons or sig- nificant digits. Someday the Whiz- Wheel and memorized math will go the way of the FSS teletype, LORAN and slide rule, replaced by systems and devices that are more accurate and user-friendly. In the meantime, we should keep some of our ancestral pilot-math skills polished, just in case we lose the newfangled elec- tronic gadgetry and need to do some hand f lyin’ and mental cipherin.’ I’m sure that our ancestors won’t mind if we use our fingers when calculating a WAG – as long as it’s not in front of the Feds, passengers or during a news conference.
Kevin Dingman has been flying for more than 40 years. He’s an ATP typed in the B737, DC9 and CE-650 with 25,000 hours in his logbook. A retired Air Force major, he flew the F-16 and later performed as an USAF Civil Air Patrol Liaison Officer. He flies volunteer missions for the Christian organiza- tion Wings of Mercy, is retired from a major airline, flies the Cessna Citation for RAI Jets, and owns and operates a Beechcraft Duke.Contact Kevin at dinger10d@gmail.com.