You can make flying planes a little easier by applying a few different rules of thumb provided below.

Vern Weiss

There’s a lot of minutia and head work involved in flying planes and sometimes a pilot can get bogged down with the calculations and mental gymnastics required. “East is least and West is best” and “Accelerate – North, Decelerate – South” come to mind. Thank goodness they came up with those to aid in flying planes, or I would still be studying for my private pilot written exam.

My particular annoyance is the Metric system’s ornery method of measuring temperature. Fortunately, some angel from Heaven was sent to give us, “Double it and add thirty.” So if I need to convert 30 degrees Centigrade to Fahrenheit it becomes 30 x 2 plus 30 equals 90. It actually comes to 86 degrees Fahrenheit so I must offer this caveat about all pilot rules of thumb: A rule of thumb is a “broad application that is not intended to be accurate or reliable for every situation. It is an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.“¹

Hydroplaning

Say, let’s go hydroplaning today! The runway is wet and we are bored so let’s inject a little excitement into a hum-drum afternoon.

Hydroplaning occurs when a boundary layer of water prevents a tire from making direct contact with a hard surface and the result can be the loss of steering control and braking. The formula for computing the minimum speed at which a tire hydroplanes would fill two pages of lined filler paper. However Professor Tom Thumb created his Rule of Thumb and shrunk the arduous calculations down to simply “the square root of the tire pressure times 9.” Thank you, Professor Thumb. (Actually, it is known as Horne’s equation.) This means that if your tire has 45 PSI in the nose and 38 PSI in the main tires, the nose will start hydroplaning at 60 knots and the mains will start hydroplaning at 55 knots. Yes, you are seeing that correctly; your mains will be hydroplaning before your nose as you accelerate and they will continue to hydroplane after your nose has stopped as you decelerate. This rule of thumb works no matter if you have air or nitrogen in your tires. It can also be applied to your automobile tires and give you an edge when the highway is wet enabling you to keep your speed below the threshold of where you’ll start hydroplaning. (i.e. tires inflated to 35 PSI even though they may be 44 PSI-rated tires will begin hydroplaning at 65 knots (74.8 MPH). This is a rule of thumb only. There are all kinds of tires, type H, radial-belted and bias-ply. Bias-ply tires used on aircraft have the highest speeds before they’ll hydroplane. Type H and radial-belted tires hydroplane at lower speeds (the formula “square root of the tire pressure times 6” should be used).

Rolling Out of a Turn

A well-known rule of thumb for flying planes you may have learned in the simulator is when to begin rolling out of a turn to straight-and-level and when to level out from a descent. A cozy, comfy roll-out from a turn is simply half your angle of bank. If your bank is 15 degrees, start your roll-out 7 1/2 degrees before your desired heading. If your bank is 45 degrees, start your roll-out 22 1/2 degrees before your heading. Make sure it is twenty-two AND A HALF degrees! Not 22-1/4 or 22- 3/8, but 22 and a half! Heh heh…I’m having a little fun with you.

As for descending, if you prefer not to undershoot…then overshoot…then dive back down like a porpoise at Sea World, start your level-out at your rate of descent divided by 10. If you’re descending at 1,000 feet per minute, start leveling out 100 feet before your altitude. Pretty simple, eh?

Crossing Restriction Clearance

When flying planes, pilots are frequently called upon to participate in The Dreaded Crossing Restriction clearance from ATC. “Pterodactyl Two-Eight-X Ray, Descend to 7,000 feet and cross 10 miles south of Earwax VOR at 2,000 feet.” H-m-m-m. So you hustle down to 7,000 because that’s where he wants you to be. Now you have to figure out when to start your descent from 7,000 to 2,000. This “rule of thumb” is predicated on ATC’s expectation that you will descend at an angle of 3 degrees which is comfortable for any aircraft. (By virtue of their speed, jet aircraft attain this rate at approximately 2,000 FPM). It’s simple. Drop the last 3 zeros of the altitude change required. Multiply this number by 3. This figure represents the number in miles prior to the crossing fix necessary to safely arrive at the new assigned altitude. In our example, you are going from 7,000 to 2,000 feet which is an altitude change of 4,000 feet. Drop the 3 zeroes to get “4.” Multiple “4” by the constant “3” to get 12. To make this crossing restriction you will start your descent NO LESS than 12 miles from the VOR. Actually, I add an extra buffer of 5 and would start my descent when I was 17 miles from the VOR. I don’t like filling out NASA reports nor do I wish to get any letters from the FAA.

Here’s a thought to ponder: Contemporary airplanes are now equipped with super-colossal computers that can figure this out for you. In fact, these FMS systems will alert you when to start down and even provide you with a virtual “glide slope” to ensure arriving at the desired point at the correct altitude. This is handy when all the data is already plugged into your FMS but what happens when it isn’t? When you’re given an unexpected crossing altitude you have the additional task of key-punching in all the data. That takes time and I have seen so many pilots in the simulator miss their crossing restrictions because they were fat-fingering buttons and trying to get their FMS programmed. Make it easy on yourself. You know your “X” miles from the VOR and the controller wants you at such-and-such altitude when you’re “Y” miles…do it in your head and it will take you less than ten seconds to compute instead of a minute or more of pirouetting your fingers around the FMS keyboard. Even when I already have the crossing restriction programmed into an FMS I still do it in my head as a double check that all the data-based algorithms are correct (and I have seen them not so).

Amount of Fuel

So you land and you gotta buy fuel so the petroleum barons can afford to own nine luxury homes throughout the world. There are two ways to go about this. You can dig your calculator out of your bag and make work for yourself…or you can take the easy way out and call on ol’ Professor Thumb.

It’s simple when your airplane registers in gallons. If you land and your 50 gallon tank is half- full and you want it three-quarters full you tell them you need 12 or 13 gallons. But what happens when you’re flyin’ with the Big Dogs and you no longer deal in gallons? Large recips and turbine aircraft generally have their fuel metering in pounds. But yet, FBOs deal in selling gallons.

There are two ways to do this: You can divide what you want by 6.79 pounds which will derive the number of gallons you need (ugh) or…you can use Professor Thumb’s Handy Dandy Instant Solution.

• A. You subtract the fuel you have in the tank from the total fuel you want to have.
• B. Divide that number in half.
• C. Then add the half (B) to the number you started out with (A).
• D. Drop the last zero.

The answer is the number of gallons to buy.

Say you landed with 1,000 pounds of fuel and you want to leave with 3,000 pounds of fuel. A= 2,000 B= 1,000 C= 3,000 D= 300 gallons

Don’t believe me? Try it in your head. You landed with 1,600 pounds of fuel and want to leave with 2,600 pounds. How much fuel do you order?

Did you say the amount in the footnote below?²  Well done.

In countries such as Canada they believe in liters and the rule of thumb for that is simply multiplying the number of gallons times 4 and that’s close enough. Fuel is quite variable and its density changes with temperature. But you’re an earnest and thorough pilot and after fueling you always check the fuel gauge before the fueler disconnects. If it doesn’t show you what you need, you add more. Duh.

If you have fuel tanks in each wing you obviously further divide your total by two and, instead of “150 gallons” it becomes “75 gallons per side” (except if the FBO has a promotion giving away something cool with a fuel purchase of 200 gallons or more then the minimum fuel order HAS to be 200 gallons).

These are just a few of the “cheats” pilots have devised to make the job of flying planes simpler. Ball-parking is acceptable so long as, when doing it, you cross check your answers with other available cues.

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References and Footnotes:

1 – Thank you, Wikipedia.

2 – 150 gallons.

^{Featured Image: Kent Wien}