Plane & Pilot
Sunday, January 1, 2006

Top 10 Rules Of Thumb

Piloting an aircraft requires decision and precision. Quick references to the basics can make both easier.

Pilots are expected to know lots of stuff. So it should come as no surprise that they like all the help they can get when memorizing, analyzing and calculating aviation concepts. This is one reason why there’s so many mnemonics and abbreviations associated with flying. Pilots are also aided with staying on top of things by the various rules of thumb. According to, a rule of thumb is “an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.”

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5. 10/20 Rule Of Weight.
Rule of thumb No. 5 states that a 10% change in weight will cause at least a 20% change in takeoff and landing distances. More weight requires more runway. This rule, too, has some variation in ground-roll numbers among aircraft. A review of Cessna 172, Piper Warrior II and Beech Duchess data shows that a 10% addition of weight yields a 22% to 25% increase in distances. Obviously, if performance is critical, you’ll need to do some calculating. Even so, both 10/20 rules steer you to consider the influences of weight and speed on aircraft performance.

4. Easy Density Altitude.
Ever see a Koch chart? It’s used to determine density altitude and it can be more than a little perplexing. Instead of mulling over yet another “spaghetti chart,” use rule No. 4. For every degree of Celsius variation from standard temperature, density altitude (DA) changes by 120 feet. Increases in temperature cause DA to go up; decreases make DA go down. There’s even a formula: DA equals pressure altitude plus 120 times the difference between actual air temperature and standard. So if you’re at sea level, the altimeter is 29.92 and it’s 25 degrees C, DA could be calculated by adding pressure altitude (zero, in this case) to 120 times the result of 25 degrees C (actual) minus 15 degrees C (standard at sea level). Crunching the numbers gives a DA of 1,200 feet.

3. Density Effects.
Wouldn’t it be nice to know what each degree temperature change does to takeoff performance (other conditions remaining the same)? Rule of thumb No. 3 steps in to answer this challenge. For each degree Celsius of divergence from standard, the takeoff roll changes by roughly 1%. According to the Cessna 172P manual, a takeoff at sea level with standard conditions would require a roll of 890 feet. Up the temperature five degrees, and the roll jumps to 925 feet, just under a 5% boost.

2. Abort! Abort!
If you haven’t heard of rule of thumb No. 2, you need to take some time to get cozy with it now. It states that an aircraft should achieve 70% of its flying speed by the time it has consumed 50% of the runway or an abort is in order. This halfway point is so important that there’s now a sign available to mark it, which has a “1/2” on it (see AIM Figure 7-5-1). You may be wondering why you need more than half your speed when you’ve only used half your runway. This is due to the fact that acceleration doesn’t occur in a linear fashion. You can actually calculate the percentage of liftoff speed required for any given distance of runway with the formula 10 times the square root of the percentage of runway used.

Grain-Of-Salt Rule.
This is probably one of the best rules of thumb out there. It reiterates the importance of skepticism by pilots in regard to what’s in the performance section of aircraft manuals. According to the Piper Warrior II manual, the performance charts “do not make any allowance for varying degrees of pilot proficiency or mechanical deterioration of the aircraft.” Thus, performance data reflects the best-case scenario and realistically is underestimated. Considering this premise, rule No. 1 rightfully declares that all performance data should have at least a 20% safety margin tacked on as insurance. If the performance required is so tight it doesn’t allow for this leeway, it may be best to rethink the situation.

It doesn’t take a rocket scientist to figure out that rules of thumb aren’t meant to replace performance charts or good judgment. They can, however, help pilots understand the influences of different performance factors on their aircraft, which should, by default, help augment safety. Whether helping measure the distance remaining to a checkpoint or preventing the continuation of a takeoff gone awry, rules of thumb can be excellent additions to the arsenal in a pilot’s mental flight bag.


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