Rules of Thumb You Should Know

1) 10% Weight Increase = 20% Increase in Takeoff and Landing Distance

While this rule is far from exact, it gets you in the ball park for a normally aspirated plane.

Obviously, when it comes time to calculate your actual performance, you’ll want to pull out your POH.

2) Estimating a Crosswind Component

There’s an easy way to calculate the crosswind component.  If the wind is 30 degrees off the runway, your crosswind component is about 50% of the wind speed.

If the wind is 45 degrees off the runway, the crosswind component is about 75% of the wind speed.

And if the wind is 60 degrees or more off the runway, the crosswind component is roughly the same as the total wind speed.

3) Takeoff roll increases about 10% for every additional 1,000 feet of density altitude

For most normally-aspirated airplanes, you can add about 10% to the takeoff roll distance for every 1,000′ increase in density altitude (DA).

For example, in Denver, with an increase of 3,200′ of density altitude, you’d increase your takeoff roll by about 32%.

So if you have a 1,500′ takeoff roll on a standard day in Denver (3o C or 37o F), you’ll increase the takeoff roll to almost 2,000′ on a 30o C (86o F) day.

4) When Should You Start Your Descent from Cruise Altitude to Pattern Altitude?

A three degree glide angle will give you a comfortable descent rate in just about any aircraft. A three degree descent equates to a gradient of 318 feet per nautical mile. Unfortunately, 318 is not a mathematically friendly number, so we’ll just use 300.

When you’re approaching an airport, you simply plan your descent point:

Divide the altitude you want to lose by 300.

For example, if you’re at 11,500 feet, and you need to get down to a pattern altitude of 2,500 feet, you need to descend 9,000 feet.

9,000/300 = 30 miles.

If you start a 3 degree descent when you are 30 miles out, you’ll hit pattern altitude as you reach the airport. You’ll want to be at pattern altitude four to five miles before reaching the airport, so let’s start down at  35 miles.

5) Descent Rate from Cruise Altitude to Pattern Altitude

You’ve started your descent at 30 miles (see #4). But, how do you know if you’re descending towards the airport on a 3o glide path? Easy peasy! Simply multiply your ground speed by 5. For instance, if you are descending at a ground speed of 150 knots, for a 3o glide path, you should have a rate of descent of 750 FPM.

 6) Approach Descent Rate

What a coincidence! It turns out that a comfortable descent angle for landing just happens to be 3 degrees. In fact, most ILS and VASI/PAPI glide slopes are set for a 3 degree glide slope.

Check your ground speed on final and multiply it by 5. This will give you a target feet per minute descent rate. For instance, if your ground speed is 80 knots, you should be descending at about 400 feet per minute.

 Conclusion

You don’t need to have Einstein’s intellect to realize that rules of thumb are not meant to replace performance charts or good judgment. They can, however, help pilots understand the influences of different performance factors on their aircraft, and perhaps save a life or an ego. Whether helping you arrive at pattern altitude at just the right point, or preventing the continuation of a takeoff that could have gone awry, rules of thumb can be excellent additions to a pilot’s mental flight bag.

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