eTold Background Information

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Last updated Sept 25, 2011

This page is meant primarily for non-aviation people who are interested in an explanation of the importance of calculating takeoff and landing peformance data.

Why etold?

On every commercial flight on a jet, the pilots must calculate takeoff and landing performance. For a given weight, temperature, elevation, runway condition (dry vs snow, etc.) and aircraft configuration, a calculation must be done to come up with a set of performance numbers (takeoff distance, speed, etc.). This is called Takeoff and Landing Data (TOLD). This must also come from FAA-approved sources like the Airplane Flight Manual (AFM).

While this applies to all jets, large transport jet companies (737's, Airbusses, etc.) have dispatch departments that have software which figures all that stuff out and gives a printout to the pilots.

Smaller modern jets with a lot of automation have computation equipment on board which figures out the TOLD data as well.

But for smaller and older business jets, like Learjets, Cessna Citations, etc., the pilots must do this calculation. For vanilla cases, no wind, etc., he can use tables that contain a set of datapoints. If the conditions don't match the datapoint in the tables, he must interpolate, often in multiple directions. And the quick tables have no wind correction, although ususually if there's wind, it'll be a headwind which means the actual performance will be better than the tables would show.

Alternatively, the long way is to go into the AFM and spend much time applying the conditions to numerous graphs in sequence to come up with the performance numbers. This takes a long time and is quite error prone. As a result, pilots don't often do this.

As an aside and precautionary note, performance isn't an exact science as you might think, because the book numbers are for a highly-proficient test pilot, clean airplane (no bugs on the wing, etc.), in other words, ideal conditions. It's like doing high-school physics with massless pulleys and frictionless planes, and then trying to apply it to real life.

So I wrote a program (etold) into which I digitized the relevant performance graphs to do the graphical analysis quickly. In addition, instead of having to type in the weather conditions and look up the elevation, runway lengths, etc., etold goes out onto the internet and finds this information and fills it in for the pilot.

The calculations are all done by mathematically interpolating piece-wise linear approximations of the graphs in the back of the AFM. Some of the contaminated runway data from the AFM are in the form of tables, and so I typed in the tables as-is and do a 3-dimensional interpolation when values are needed between given values in the tables.

Program Screenshots on the Palm Pre

I've put the screen-shots on the web and provided URLs (links).

I've currently implemented the program on the Palm Pre smart-phone. The program has four basic screens (due to the small size of the phone):

However, note that all the numbers are bogus for these screen shots because I'm using the emulator which does not do actual calculations. But it does show you what the screen looks like. The phone has properly-calculated numbers of course.

The first screen you see when you start the program (after the disclaimer) is the Dep Wx screen, I have 3 screenshots illustrating the screen as you scroll down:

In the top screen you type in the airport and tap on "Find Apt" on the bottom left. This will cause the phone to go onto the internet to government databases to get the METAR and TAF (weather conditions) for which you can see the raw data in the bottom shot. The program parses the METAR data and puts the numbers in the proper places in the top screen - wind direction and speed, temperature, etc. You can also override any values you want manually, so if you aren't leaving for a few hours and you think it'll be 4 degrees C hotter, you can plug in that number.

etold will also go out and find the airport elevation and the set of runways (directions, lengths, etc.) for that airport. You can then select which runway you want to use, and say what kind of contamination it has (dry, wet, ice, etc.). You have to put the slope in manually.

You also set whether you'll take off with anti-ice off/nacelle/on, anti-skid off/on, and what kind of engine nacelle you have - normal or Aeronca reversers. I don't have any data for the Dee Howard ones.

Then you go to the "Dep Vs" screen to calculate the departure data:

Select Flaps 8 or 20, and you can use the slider to select your takeoff weight. etold then calculates all the numbers.

It also tells you how much fuel you must dump to get down to landing weight for an emergency return. If you zero the dump, it'll still tell you the correct Vref and landing distance for your overweight landing.

It also calculates your takeoff weight limit if the AFM graphs show you to be limited to something lower than the normal takeoff weight due to temperature.

Then you go to "Arr Wx" and select your arrival weather the same way you selected your "Dep Wx". I didn't include a screen shot of this because it's the same as the "Arr Wx" screen. Note you can select what the runway contamination is for the arrival runway on this screen as well.

Then you go to the "Arr Vs" screen to calculate your landing data:

You select your fuel burn (or dump) which determines your landing weight. Select what regs you're flying under (91, 135, or 135-low-visibility). Select your flap setting. etold then calculates all the numbers you see in the screen shot.

And again, etold calculates your landing weight limit if the AFM graphs show you to be limited to something lower than the normal landing weight due to temperature.