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Performance on FAA Tests and in the Real World – Part 1

June 29, 2012/in Tips and technique/by studentpltnews

Pressure and Density Altitude on FAA Tests and in the Real World One of the things that you must learn as a pilot is how to determine the approximate performance you can expect out of your airplane on a given day. I say the “approximate performance” because there are simply too many variables that will […]

What Your Aircraft Feels

June 4, 2012/in Personal stories/by studentpltnews

A few years ago, at one of the local airports, there was an aircraft accident. Two guys took off in their Grumman Tiger on a beautiful, summer afternoon. The sun was shining and the winds were calm. They lifted off, barely clearing the fence at the end of the 3,700 foot runway. The pilot then […]

Performance on FAA Tests and in the Real World – Part 1

June 29, 2012/in Tips and technique/by studentpltnews

What Your Aircraft Feels

June 4, 2012/in Personal stories/by studentpltnews

Full Sized Blog Element (Big Preview Pic)

Performance on FAA Tests and in the Real World – Part 1

June 29, 2012/in Tips and technique/by studentpltnews

Pressure and Density Altitude on FAA Tests and in the Real World

One of the things that you must learn as a pilot is how to determine the approximate performance you can expect out of your airplane on a given day. I say the “approximate performance” because there are simply too many variables that will affect our airplane’s performance for which we can’t directly account to get exact performance.

Pressure and Density Altitude calculations are key factors in determining performance but these can be confusing. Also confusing is the FAA’s emphasis on precise calculations on the FAA knowledge test when these values are only a part of the total performance equation. I’ll cover some of the other values in the equation in another blog post. In this post, let’s take a look at Pressure and Density Altitude.

The FAA defines Pressure Altitude as the altitude above the standard 29.92″ Hg plane. While Density Altitude is the Pressure Altitude corrected for nonstandard temperature.

Density Altitude is used in computing the performance of an aircraft and its engines. Some manufacturers utilize Pressure Altitude on their performance charts but they address the nonstandard temperature within the chart. In this way, they are actually using Density Altitude but the pilot isn’t required to calculate it.

Variation in Pressure and Density Altitude Calculations

There are several ways that Pressure Altitude can be determined depending on the accuracy and precision required by the calculation.

FAA knowledge test supplements include a table on the “Density Altitude Chart” that shows a “Pressure Altitude Conversion Factor.” The value obtained from this chart is based upon the altimeter setting and is added to the indicated altitude (the correction value will be negative when the altimeter setting is higher than 29.92″ Hg). This correction value is precise to the nearest foot and provides a reasonably accurate measure of Pressure Altitude.

The Density Altitude value obtained from this chart has some minor accuracy issues when using the standard temperature line. The lower anchor point of the standard temperature line is not set at 15° C for sea level as it should be. This lower anchor is set at approximately 16° C. This error in the chart should have less than 100 feet of error with regard to calculations.

There is a precise and rather involved formula for computing Pressure Altitude with exceptional accuracy. It can be used to calculate the Pressure Altitude out to several decimal places if needed. In general, aviation calculations involving POH values and used airplanes do not require this level of precision or accuracy.

If you are at an airplane, Pressure Altitude can be determined by setting 29.92 in the Kohlsman window and reading the altitude from the altimeter.

Many people calculate Pressure Altitude using the rule of thumb that 1″ Hg change equals 1000′ of altitude change. This rule of thumb is accurate enough for most aviation calculations, as long as you know its limitations. The rule assumes a linear change in pressure with altitude. This assumption is not correct but the answer doesn’t stray far from the truth when the values being calculated are below 10,000′ MSL. The rule, while easy to use, also has a minor inaccuracy as such that 1″ Hg change is not exactly 1000′ of altitude change but this issue should produce less than 100 feet of error per inch of change.

Density Altitude calculations have many of the same issues. In aviation, we use Pressure Altitude and temperature to calculate Density Altitude. Yet, in reality, humidity also has an impact on Density Altitude since water molecules in gaseous form are lighter than the typical molecules in the atmosphere. The more gaseous water (higher humidity) found in the atmosphere, the less dense the air and thus the higher the Density Altitude.

Why do we ignore humidity in common aviation calculations of Density Altitude? Apparently, it was decided at some time in the past that it doesn’t make that big of difference with regard to the performance of the airplane for practical purposes.

Performance Calculations

This brings us back to why we compute Pressure Altitude and Density Altitude in the first place. We want to know how the airplane is going to perform on a given day.

FAA test questions would have you believe that you should calculate your performance to the nearest foot. As most experienced pilots will tell you, this is not grounded in reality. POH performance numbers were developed with a brand new airplane being flown by a test pilot. Do you want to bet your life on a calculation with no error margin in your 5 (or 25) year old airplane being flown by a newly minted Private pilot?

The truth is, you need a safety factor.

One way to build in a safety factor and ease your need to calculate performance to the nearest tenth of a foot is to always round the value. Ignore the rounding rules that you learned in school and round to the next safest value. For example, if your Density Altitude calculation tells you that the Density Altitude for the day will be 3213.65 feet, use 3300 feet or better yet 4000 feet when you go to your POH to calculate your takeoff and landing performance.

You may notice some minor difference in the output for Pressure and Density Altitude when you compare various charts and models of E6Bs. This variation is likely due to the method that the manufacturer selected to compute these values. These differences are generally not significant for practical purposes and should not be a concern.

For the less than practical FAA tests, these differences may mean that you won’t get the exact answer as the FAA. Just use the closest answer and you should be fine. Don’t become overly concerned about this minutia; there are other minutiae on the tests that should have your attention.

In the real world, you are better off rounding up to the next safest value. This does away with things like interpolation on tabular data. It is also a simpler and safer practice.

Keep it simple and stay safe!

What Your Aircraft Feels

June 4, 2012/in Personal stories/by studentpltnews

The pilot decided to land in the field underneath him. After quickly reducing the power to idle, he put the plane on the ground. The plane was out of control. He struck a wing and flipped upside down. The pilots struggled to open the canopy before the plane would catch fire. The good news is they escaped with minor injuries. They were lucky though; it could have ended much worse.

After the accident, the pilot stated he didn’t understand what was wrong with the airplane. Was it an engine problem? Why did it take so much runway to lift off the ground? Why couldn’t they clear the trees way off the end of the runway? He and his flying buddy had made successful takeoffs every flight in the months leading up to this accident.

But as I mentioned before, unfortunately, it was a beautiful, hot, early summer afternoon. The density altitude was relatively high (around 3,000 feet) and there was no wind to “help” the airplane off the ground. The aircraft was near max gross weight as they usually are. They didn’t understand the dramatic effect that the elevated density altitude would have on their takeoff distance. It also decreased their climb performance which is reflected in their inability to clear an obstacle. This lack of appreciation and understanding almost cost them their lives.

Density altitude is pressure altitude corrected for non-standard temperature. Of all your various altitudes that you’ll learn about (absolute, true, or pressure), density altitude is the most important when it comes to aircraft performance. It’s the altitude your aircraft “feels” while flying. The aircraft feels or behaves as if it were at its density altitude because hot days with high density altitude adversely affects the performance of the wings, the propeller, and the engine (rule of thumb: add about 12% to your takeoff distance for every 1000 feet density altitude, about 36% for the flight we’ve discussed). Added to this, the pilots had no headwind to help decrease their takeoff distance (10 knot headwind decreases distance by about 10%), and they were taking off uphill (1% upslope increases takeoff distance about 8%). You can see how this adds up.

Even though the pilots had made this takeoff successfully in the fall, winter, and spring leading up to the accident, they did not account for that hot, 3000 foot density altitude day at max gross weight on an upsloping runway with no headwind to help them. This resulted in a much longer takeoff roll with decreased climb performance. They should have recognized the airplane wasn’t performing nearly to what it had for them on past flights and aborted the takeoff.

With summer right around the corner, it only takes a few minutes to check your performance calculations. See how the hotter temperatures affect your airplane’s performance in all phases of flight. Note that most operating handbooks base their takeoff and landing distances on shortfield technique. If you plan a normal takeoff roll, the numbers in the POH will not work. Also, try to plan your flights earlier in the day when you can take advantage of the cooler temperatures and better performance.

Half Sized Blog Element (Single Author Style)

Half Sized Blog Element (Multi Author Style)

Performance on FAA Tests and in the Real World – Part 1

What Your Aircraft Feels

Performance on FAA Tests and in the Real World – Part 1

What Your Aircraft Feels

Full Sized Blog Element (Big Preview Pic)

Performance on FAA Tests and in the Real World – Part 1

Pressure and Density Altitude on FAA Tests and in the Real World

Variation in Pressure and Density Altitude Calculations

Performance Calculations

What Your Aircraft Feels

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