Can someone define AGL?

User Support Hub Aircraft & Systems
1

I’m trying to fly this simulator as realistically as possible and I’m running into a concern over flying correctly. Perhaps there’s some RL pilots that can help me out.

AGL is Above Ground Level. However, when flying at a set altitude what is considered AGL? Let’s not get into the highest structure in a grid and avoidance either vertically or horizontally; I’m talking about flying cross country over large, sparsely populated areas.

I’m in Canada and Canadian Aviation Regulations state that, above 3,000’ AGL, one must fly a powered aircraft:

Magnetic Track 0-179 – Odd thousand feet for IFR and add 500 feet for VFR. e.g., IFR/VFR – 3,000’/3,500’; 5,000’/5,500’; 7,000’/7,500’; to a maximum altitude of 17,500’ for VFR.

Magnetic Track 180-359 – Even thousand feet for IFR and add 500 feet for VFR. e.g., IFR/VFR – 4,000’/4,500’; 6,000’/6,500’; 8,000’/8,500’; to a maximum altitude of 16,500’ for VFR.

When I’m flying East I like to fly about 5,500’ AGL and flying West at about 4,500’

I’ve read an awful lot of the CAR and nowhere can I find the definition of “Above Ground Level” or how to fly across fluctuating terrain.

Here’s an example to show why I’m asking this:

I fly VFR out of CYKF Region of Waterloo International Airport with an elevation of 1,054’. I flew a trip from there to CYEL Elliot Lake travelling around Georgian Bay. Along the way the land elevation fluctuates from a high of about 1,650’ ASL (Above Sea Level) to a low of about 600’ ASL. As you can see, if I set my altitude for either of those extremes I’m going to be flying into an inappropriate altitude. If I take the average I’d be flying into IFR territory.

So, my question goes back to, “How do you fly AGL?”

Thanks for any feedback.

Bob

2

The IFR and VFR cruising altitudes are in ASL, not AGL. and they only come into effect when you’re above 3000 AGL. If terrain is at 2000 ASL, then the cruising altitude guidelines of anything less than 5000 ASL do not apply because they’re less than 3000 feet above the ground. The height above ground level doesn’t have to be exact though, especially if terrain is changing alot. No one’s gonna have a problem if you’re not quite at or above 3k AGL and still following the cruising altitude guidelines. Really you could follow them below 3000 AGL if you really wanted to, but it would become difficult due to the terrain and structures, since you need to maintain 1000 feet above the highest structure within 2000 feet horizontally of the aircraft.

I believe (may want to fact check me on this) that the CARs always state altitude in ASL unless otherwise noted, to avoid any further confusion.

3

In short, you don’t “fly AGL”.

Regulations & guildelines are written in this way (AGL) is so that they can be applied across the whole airspace/country in an easy-to-understand format.

The Canadian regulation you quoted is very much set with gliders in-mind. Their instruction is simply to not soar 3000 above the ground. If this regulation was set with anything other than AGL, it would become very, very confusing.

4

ATC in game will make you fly AGL in any visual approach. It sends to you commands to climb and descend every few seconds following terrain elevations until you crash into a mountain :sweat_smile:

I think AGL has only real application in the military scope, for tactical low level flying or nose on earth. In the case of helicopters for instance, where you even fly closer to terrain, radar altitude is relevant and many accidents happened due to extreme low flying at high speeds.

Cheers

5

Two different things discussed here confusing AGL with flight levels. As in Canada, the US has the same flight level structure and the lowest usable level is 3000 IFR 3500 VFR MSL unless constrained by the published Minimum Obstacle Clearance Altitude (MOCA) or Minimum Enroute Altitude (MEA). Separately, not all flying is done at flight levels such as most VFR GA flight and the minimum legal altitudes for this is 500 ft over non populated areas or 1000 ft over populations and these are AGL altitudes. So one can legally fly along a coastline at 500 ft AGL IF this coast is not along a populated area and the airspace is not otherwise restricted.

6

On this page in the CAR there is only one reference to ASL and that is in section 602.32 subsection (1)a - speeds above 250 knots.

There are only two references to AGL:
Section 602.32 subsection (1)b - speeds above 200 knots.
Section 602.34 subsection (2)a - altitudes appropriate to the track.

And, again, I can’t find a regulation stating how two or more aircraft approaching an area in which conflicting tracks with appropriate altitudes would avoid each other.

I’m leaving what I just wrote here for future reference for someone else but, oh boy! Have I just had a face-palm moment!

As long as I’m flying high enough to clear the HIGHEST ground by 3,000’ or more then whatever appropriate altitude for track I’m flying has nothing to do with the undulating ground beneath me because the altimeter IS IN ASL.

Sometimes I over-think too much. I’d ask someone to slap me upside the head but I’ve already left enough bruises myself.

Sorry for the distraction.

Bob

7

Communications!
(d) Arrival(CAR602.101)
(i) Report before entering the MF area and, where circumstances permit, shall do so at least five minutes before entering the area, giving the aircraft’s position, altitude and estimated time of landing and the pilot- in-command’s arrival procedure intentions;
(ii) Report when joining the aerodrome traffic circuit, giving the aircraft’s position in the circuit;
(iii) Report when on downwind leg, if applicable;
(iv) Report when on final approach; and
(v) Report when clear of the surface on which the aircraft has landed.

8

Look at the VFR charts, there will be a minimum safe altitude in MSL (mean sea level), you must stay above this when flight planning enroutes.

IMG_3702
IMG_37021542×1209 233 KB

Notice the 97 and 58 in the two sectors? You would plan you height based on those. For the left sector, plan 10500 west, and 11500 east as an example. And 6500 west/ 7500 east for the right sector.

9

But bear in mind that above 10,000 feet for more than 1/2 an hour requires oxygen. Also, as @BubblyDruid56 said above, you don’t have to fly above 3000 feet AGL. Below 3000’ AGL, you don’t have the prescribed cruising altitudes. In the case of the mountainous sector on the chart above, one could fly below the height of the highest terrain by following the valleys, in order to preclude the need to use supplemental oxygen.

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10

AGL is what is says it is…

It is very much a useful concept, particularly when flying helos, or mountain flying (when you will certainly be under MSA) when you will constantly adjust your altitude to maintain the desired separation from the terrain - not just military nap-of-the-earth flying. The sim is very good for this as the terrain is rendered well enough to develop good visual cues for your height AGL without need for a radio altimeter.

11

I might have misunderstood the point, thought that @AerobaticAce was looking for something like chapter 3.2 (3.2.2) in ICAO Annex 2.

12

*12,500 over 30 min.

13

A little OT but I’ve spent a good bit of my career working with both aviation and FCC data and I’m surprised by how many people get confused with AGL/AMSL outside of pilots and RF engineers.

I’ve also noticed Europeans (or former UK Europeans) that speak proficient English have a hard time as well.

14

This discussion is about the Canadian regulations. In canada it is 30 minutes between 10,000 feet and 13,000 feet. Above 13,000 feet, you need to be on oxygen all the time.

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15

Just to add European regulation:
European commision loosened the regulation, EASA NCO (non-commercial, non-complex operations) defines the following:

„‘NCO.OP.190 Use of supplemental oxygen

(a) The pilot-in-command shall ensure that all flight crew members engaged in performing duties essential to the safe operation of an aircraft in flight use supplemental oxygen continuously whenever he/she determines that at the altitude of the intended flight the lack of oxygen might result in impairment of the faculties of crew members, and shall ensure that supplemental oxygen is available to passengers when lack of oxygen might harmfully affect passengers.

(b) In any other case when the pilot-in-command cannot determine how the lack of oxygen might affect all occupants on board, he/she shall ensure that:

(1) all crew members engaged in performing duties essential to the safe operation of an aircraft in flight use supplemental oxygen for any period in excess of 30 minutes when the pressure altitude in the the passenger compartment will be between 10 000 ft and 13 000 ft; and

(2) all occupants use supplemental oxygen for any period that the pressure altitude in the the passenger compartment will be above 13 000 ft.’;“

So generally no hard altitude requirements (a). The recommendation in (b) was a hard requirement before this change in 2016.

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16

Aaaaah. Meters.

(:point_up_2:t3:Joke)

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17

:grin: I see your smart alec joke and raise you one:

metres.

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18

OP. You are using a Computer of some type… why not just GOOGLE a question like this, and get a Reliable Answer, rather than expecting others to type a long explanation,( that may be correct or incorrect) to something that is so easy to get an “reliable” answer to with a Search.

Do you really expect someone to spend their time, and give you a full answer, like

Aviation

A pilot flying an aircraft under instrument flight rules (typically under poor visibility conditions) must rely on the aircraft’s altimeter to decide when to deploy the undercarriage and prepare for landing. Therefore, the pilot needs reliable information on the height of the plane with respect to the landing area (usually an airport). The altimeter, which is usually a barometer calibrated in units of distance instead of atmospheric pressure, can therefore be set in such a way as to indicate the height of the aircraft above ground. This is done by communicating with the control tower of the airport (to get the current surface pressure) and setting the altimeter so as to read zero on the ground of that airport. Confusion between AGL and AMSL, or improper calibration of the altimeter, may result in controlled flight into terrain, a crash of a fully functioning aircraft under pilot control.

While the use of a barometric altimeter setting that provides a zero reading on the ground of the airport is a reference available to pilots, in commercial aviation it is a country-specific procedure that is not often used (it is used, e.g., in Russia, and a few other countries[which?]). Most countries (Far East, North and South America, all of Europe, Africa, Australia) use the airport’s AMSL (above mean sea level) elevation as a reference. During approaches to landing, there are several other references that are used, including AFE (above field elevation) which is height referencing the highest point on the airfield, TDZE (touchdown zone elevation) or TH (threshold height) which both refer to the elevation of the landing end of the runway measured AMSL and AGL respectively.

In general, “altitude” refers to distance above mean sea level (MSL or AMSL), “height” refers to distance above a particular point (e.g. the airport, runway threshold, or ground at present location), and “elevation” describes a feature of the terrain itself in terms of distance above MSL.[2][3] One mnemonic that can be used is: if it is an altitude you can fly there, if it is an elevation you can walk there, and if it is height that’s how far a rock will fall before it hits the ground.

Atmospheric sciences

In weather and climate studies, measurements or simulations often need to refer to a specific height or altitude, which is naturally AGL. However, the values of geophysical variables measured in various places on the natural (ground) surface may not be easily compared in hilly or mountainous terrain, because part of the observed variability is due to changes in the altitude of the surface. For this reason, variables such as pressure or temperature are sometimes ‘reduced’ to mean sea level.

In general circulation models and global climate models, the state and properties of the atmosphere are specified or computed at a number of discrete locations and heights. When the topography of the continents is explicitly represented, the altitudes of these locations are set above the simulated ground level. This is often implemented using the so-called sigma coordinate system, which is the ratio of the pressure at a location (latitude, longitude, altitude) divided by the pressure at the nadir of that location on ground surface (same latitude, same longitude, altitude AGL = 0).

Broadcasting

In broadcasting, altitude AGL has relatively little direct bearing on the broadcast range of a station. Rather, it is HAAT (the height above the average terrain (in the surrounding area)) which is used to determine how far a broadcast station (or any other sort of VHF or higher radio-frequency) transmission will travel.

From aviation safety perspective though, the more important aspect is the height of the radio tower used to support the radio antenna. In this case, height AGL is the only important measurement for aviation authorities, which require that some tall towers have proper Aircraft warning paint and lights to avoid collisions.

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19

This discussion reminds me of the axiom about reading obstacle heights on a sectional. “The top number is what your altimeter will read when you hit the obstacle (MSL) and the second number tells you how far you’ll fall to the earth after you hit it (AGL)”.

obstructions

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Brilliant!