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 Wikipedia.com, 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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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 Wikipedia.com, a rule of thumb is “an easily learned and easily applied procedure for approximately calculating or recalling some value, or for making some determination.”

Pilots can use these approximations, which deal mostly with operations and performance, as shortcuts to information that normally requires much more drudgery to uncover. It’s important to note that while aviation rules of thumb often provide answers that are almost identical to those produced longhand, they are, in fact, estimates. If you find that aircraft performance or the like is particularly critical, it’s best to use charts and computation in lieu of a rule of thumb. With that said, there are many excellent rules of thumb out there. Let’s take a look at the most utilitarian.

Top 10 Rules Of Thumb

10.
A True Rule Of Thumb.

What good is a rule of thumb if you can’t really use your thumb? Well, believe it or not, your stubby finger is good for something other than hitchhiking. For the average individual, the length between the tip of one’s thumb to its midpoint (the knuckle where it bends) equates to about 10 nm on a sectional chart. This can be helpful when eyeballing distances, such as for a quick deviation, although it’s not recommended to use this method to measure an entire route or to stay clear of unfriendly airspace.

9. Avoid Being Crossed.
Many a headache has been caused by the stress over how much crosswind component exists for a particular flight. Even more throbbing ensues upon pulling out the age-old crosswind chart. There’s an easier way! If the wind differs from the runway heading by 15 degrees, the crosswind component is one-quarter (25%) of the wind velocity. If the difference between the wind and runway is 30 degrees, the crosswind is half of the reported wind speed. If the wind makes a 45-degree angle with the runway, the crosswind component is three-quarters (75%) of the overall wind speed. And when the windsock is pointing 60 degrees or more from the runway centerline, just assume the crosswind is the same as the total wind (it’s pretty close, and you’d only be overestimating the crosswind component, which is probably a good thing anyway).

8. Starting Down.
One thing pilots of all experience levels struggle to grasp is when to start down from cruise. I remember riding in the jumpseat of a regional jet while the pilot flying was having a bad day determining when to descend. We ended up overhead the airport at several thousand feet, i.e., a bit high. Knowing when to start down so the descent remains at a reasonable rate is a critical piece of information, regardless of the type of aircraft flown. In most circumstances, it’s smart to plan on a three-degree descent, which equates to a gradient of 318 feet per nautical mile (the problem is that 318 isn’t a mathematically friendly number).

The descent rule of thumb is used to determine when you need to descend in terms of the number of miles prior to the point at which you desire to arrive at your new altitude. This is accomplished by dividing the altitude needed to be lost by 300 (clearly a much more pleasant number to work with). So let’s say you’re cruising at 7,000 feet and you want to get down to a pattern altitude of 1,000 feet. The altitude you want to lose is 6,000 feet, which when divided by 300 results in 20. Therefore, you need to start your descent 20 nm out (of course, you’ll want to leave some extra room so that you’re at pattern altitude prior to the proper entry, as applicable). The beauty of this rule of thumb is that you can use it to determine visual descent points (VDPs) as well. Just divide the height above threshold by 300, and you’ll get a VDP in miles from the runway.

7. How Fast To Descend?
While it’s nice to figure out when to descend, that’s only part of the picture. It’s also necessary to know what rate of descent (ROD) to use. Consequently, rules of thumb No. 8 and No. 7 go hand in hand. To determine ROD for a three-degree path, simply multiply your groundspeed by 5. If you’re going 120 knots, your ROD to fly the desired path would be 600 feet per minute (120 x 5 = 600). This makes sense. In No. 8, it was determined that a descent should be initiated at 20 nm to lose 6,000 feet. If the groundspeed is 120 knots, that means the aircraft is zooming along at 2 nm per minute. So to go 20 nm, it will take 10 minutes. Ten minutes at 600 feet per minute means you’ll lose 6,000 feet. Voilà!

6. 10/20 Rule Of Speed.
Rule of thumb No. 6 deals with speed and ground roll for both takeoff and landing. If you increase your groundspeed by 10%, your ground roll will increase by at least 20%. The actual amount the ground roll will change varies among aircraft (thus, the words “at least” have been emphasized). According to the Cessna 172P takeoff and landing charts, “for operation with tailwinds up to 10 knots, increase distances by 10% for each two knots [of wind].” A 10% change in groundspeed, which would be about five knots, brings forth an increase in ground roll of 25% (unmistakably more than the rule’s 20%). In general, though, if you fly too fast, you’ll land long.




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