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2022-12-08 12:13:54 By : Ms. AVA JIA

By William E. Dubois · November 17, 2022 · 5 Comments

Lisa, sitting in the right-hand seat, gazed out the window at the terrain below and asked: What do you suppose our AGL is?

For my non-pilot and new pilot readers, that stands for Above Ground Level, and is one of two types of altitude measurements we use in aviation.

The other is MSL, short for Mean Sea Level, which is a standardized elevation above the average level of the world’s oceans.

You use MSL all the time, probably without even knowing it.

If a coworker tells you she climbed a 14,000-foot peak in Colorado on her vacation, she’s talking MSL. When you look at any kind of map that has altitude noted, you’re looking at MSL.

And you actually see MSL every time you drive, because that’s the altitude that is the “elevation” listed on the city limits sign found on the outskirts of pretty much every city, town, village, and hamlet in the United States.

AGL, on the other hand, is the measurement of how high the airplane is above the terrain. Unlike the fixed MSL, AGL is a variable number that changes with the rising or falling MSL elevation of the ground below and flight path of the airplane through the sky above.

Now back to Lisa. I looked below, did some quick mental math, and then told her, “About six feet, I guess.”

Not to worry, we weren’t doing one of those Top Gun Maverick moves over a dry lake bed. We were in a minivan midway through the 1,200-mile commute from our New Mexico home base to AirVenture — and this is the kind of conversations two bored pilots have while driving to a fly-in.

“Although…” I continued, as I thought about it a bit more, casting another quick eye out to the pavement zipping past beneath us at an un-airplane-like speed, “I guess it depends on if you are measuring from your head, your butt, or your feet. Because the numbers are different in each case.”

“True, that,” said Lisa. And after a brief pause, “so where is AGL measured from, for reals?”

To which I realized I had no idea and had never thought about it before.

But it’s an interesting question, because we are in a new age of AGL.

While the concept has been around since Orville Wright’s flight, it’s only in recent years that we have affordable ways to measure it precisely in the comfort of a GA cockpit. Nowadays pretty much every airplane has some sort of gadget onboard that gives the pilot AGL information.

Somewhat counterintuitively, modern AGL is derived from global positioning satellite (GPS) data. Yeah, measurements taken from outer space are used to figure out how many feet above the ground you are. It’s weird, but welcome to the modern age.

In a GA airplane equipped with glass avionics, the avionics suite gives the pilot the AGL numbers. For the rest of us, the most common source is from an iPad suckling satellite data from either an installed or portable GPS antenna.

I have traditional steam gauges in my Ercoupe, so I use an iPad mounted high on my panel for navigation, which also gives me AGL data, which appears uncannily precise.

I can be flying over ragged canyons and the AGL readout on my FlightPad jumps up and down as rapidly as the terrain varies below. It shows me that the ground is 453 feet below as I cross the rim of a canyon, then the readout changes with the speed of a slot machine as the terrain drops away below me — the altitude number increasing as the ground drops away. Now 527… then 650… 812… then back up to 592 as the terrain begins to rise on the far side of the canyon.

Oh, my bad. I misspoke. I meant to say 1,453 feet below on the rim. Now 1,527… then 1,650… 1,812…

Most pilots think of AGL as the distance between either the belly of the plane — or the bottom of the landing gear — and the ground. I know I do. After all, AGL is a measure of how much sky we have to fly in, right?

But if “our end” of the tape measure is not the bottom of the airplane, but at the GPS antenna on the airplane’s back (feet vs. head), that number could be off by quite a bit.

In my case, “Tessie’s” GPS antenna is 4 feet, 9 inches off the floor of the hangar. And due to the back-sloping fuselage, my iPad is actually mounted higher than the antenna. If I’m thinking that my AGL is from my landing gear, but it’s actually from my panel-mounted iPad, my altitude readouts are off by more than five feet! And that number would be even greater with airplanes like the Cirrus, which stands higher off the ground.

Of course, if you are really low enough that it would make a difference where AGL is measured from, you’ve probably broken at least two FAA regulations, probably more.

But still, for the sake of knowledge, it is an interesting question.

So I reached out to Garmin, which makes a boat-ton load of GA avionics, as well as one of the two most common apps used by GA pilots, and asked if the company’s products measure AGL from head, butt, or feet.

Actually, I didn’t ask it quite like that. I asked — in the case of the app — does the measurement come from the GPS antenna, the box in the plane, or the iPad. In the case of the company’s installed glass systems, where do they measure from?

It turns out, the answer is more variable than AGL itself.

Jan Mackenzie, Team Leader Aviation Product Management for Garmin, who probably had better things to do, worked me though it, saying that the Garmin Pilot app “displays AGL from the highest fidelity source, beginning with the certified avionics, followed by portables, and then the iPad device itself.”

So it depends on how you are set up where your measuring tape is placed.

For installed avionics, it’s even more complicated, according to Mackenzie. The flight deck avionics process signals from the GPS antenna, but includes an offset figure chosen by the airplane’s manufacturer or the certified avionics dealer.

Basically, the magic box uses GPS antenna measurements, the offset number, and terrain data to calculate the airplane’s AGL.

So depending on your airplane, it turns out that AGL could be measured from your head, your butt, or your feet.

William E. Dubois is a NAFI Master Ground Instructor, commercial pilot, two-time National Champion air racer, a World Speed Record Holder, and a FAASTeam Representative.

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Fun article… and I think it might be good to mention the FAA reg that references “within a horizontal radius of 2,000 feet of the aircraft”… Soooooo… with respect to regulations, horizontally from your butt, passenger’s butt, or wingtip also matters.

“And you actually see MSL every time you drive, because that’s the altitude that is the “elevation” listed on the city limits sign found on the outskirts of pretty much every city, town, village, and hamlet in the United States.”

Well not really, depends on what part of the US you live in.

There is a similar issue with an ILS glideslope. What part of the airplane is 200 feet above the threshold at DH on a 200 foot ILS? Same for an LPV. I guess it depends on where the GS antenna or WAAS antenna is located relative to the landing gear.

For a Cherokee 140 or Cessna 172 it doesn’t matter that much. On an Airbus A380, it might matter!

In other words: “It depends.” Great article, thanks. Regards/J

Pilot and Rush fan says

This is both educational and entertaining. It is a superbly written answer to a question that only nerds—and likely only aviation nerds at that—have pondered and even, perhaps, debated because, well, nerds gonna nerd. Whether by car or by plane, Mr. Dubois and Lisa would be fun companions. (I also can’t help wondering—or wishing—about a possible connection between the author and another famous Dubois writer, Pye.)

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