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Interactive Challenge: How High is the Ground?

The key to understanding what airspace boundaries look like is 'seeing' what the terrain looks like...The key to understanding what airspace boundaries look like is 'seeing' what the terrain looks like. Last week we saw that the color code of a sectional chart indicates the ground's changing elevation. But there are other ways to determine how high the ground is.
Figure 1
LEGENDS AND TREASURES
Aviation charts contain legends to translate their codes, because within those codes there are treasures of information. The easiest way to determine the terrain's elevation is to look at an airport's data block. Figure 1 is the chart area around Taylor County airport. The airport's data block information is printed just to the east of the airport symbol.
  • The first line of the data block is the airport name and three-letter identifier.
  • The second line indicates that this airport has an Automated Weather Observing System (AWOS) level 3 broadcasting on frequency 121.125.
  • The third line starts off with the airport's field elevation of 921 feet above sea level. That means that while taxiing around on the ground at Taylor County a properly set and accurate altimeter should read 921 feet.
Class G airspace is on the surface and extends up to 700 feet above the surface -- this is signified by a magenta shaded area around the airport.
Class E airspace would start where Class G stops and extend on up to 18,000 feet MSL.
So the boundary between Class G and Class E airspace would be 1,621 feet MSL (921 field elevation + 700 more feet above that = 1,621 feet). Because we know the ground elevation is 921 we can calculate that we will pass from Class G to Class E airspace as our altimeter climbs through 1,621 feet over the Taylor County airport.

By knowing the ground elevation we can 'see' the overlying airspace.

Also on figure 1 and located south of the Taylor County airport is the Green River Lake. The lake appears to be formed by the dam that is shown on the west side of the lake. The note on the chart however tells us more than just the name of the lake. Below the words 'Green River Lake' we see the number 675. This is the water surface elevation of the lake. Is this important information to have? It is if you are a seaplane pilot. To a seaplane pilot, lake elevation is the same thing as airport elevation is to a dry-land pilot. And even more important is that 675 feet is the MSL height of the Earth's surface. With this information we could calculate the altitude of the boundary between controlled and uncontrolled airspace over the lake.

OFF-AIRPORT GROUND ELEVATION
The airport data block and lake elevation information that are given on the chart make finding how high the ground is easy -- you just read the number. But there are other ways to figure out how high the ground is. Look back at figure 1 and notice the tower symbol located northwest of the Taylor County airport near the town of Saloma. The symbol shows the number 1,205 in bold blue and the number 315 in parenthesis. Can we determine the airspace from just this information?

Question: You and I are flying a small general aviation airplane during the daytime. Our properly set and accurate altimeter reads 2,000 as we fly directly over the top of that 1,205 foot tower. To make the question more interesting, lets say that the flight visibility as we fly over the tower is only 2 miles due to summer haze. Are we flying legal VFR?

Answer -- Part 1: The regulations (91.119) says that in 'other than in congested areas' we must stay at least 500 feet away from a structure. We are flying 2,000 MSL, as read off our altimeter, and the top of the tower is 1,205 -- we are 795 feet above the tower and we are legal... as far as distance from the tower goes. But that is only part of the question. The real question here is: are we or are we not in controlled airspace?

Answer -- Part 2: If we are in uncontrolled airspace (Class G) then all we would need is 1-mile visibility and we would be legal. On the other hand, if we are in controlled (Class E) airspace we would not be legal because we would then need 3 miles of visibility and we currently only have two. To answer the question you must know where the boundary line between Class G and Class E is located and compare it to your altitude. So... How high is the ground? Check out figure 2.

Figure 2
The top of the tower is 1,205 feet above sea level. The tower itself is 315 feet tall, so therefore the ground that the tower is built on must have an elevation of 890 feet MSL (1205 – 315 = 890). The tower is built on the surface in an area that is inside the magenta shading that surrounds the Taylor County airport. Class G airspace inside the shading starts at the ground and extends upward to 700 feet AGL. That means that over the tower the boundary between Class G and E is located at 1,590 feet (890 + 700 = 1590). The boundary is down at 1590, but in the example we are flying at 2,000 feet MSL. That means that we are 410 feet higher than the boundary and up in Class E airspace. Class E airspace requires 3-miles visibility but we only have two! You and I would not be flying legal VFR and I would be saying 1) 'lets get out of controlled airspace' and 2) '...now, you are the PIC on this flight, right?'

THE INTERACTIVE CHALLENGE
iPilot stands for 'interactive' pilot, so lets see just how interactive we can be. I'll ask one more question about the airspace in figure 1, but this time I will not give the answer. You must answer the question by making your own vertical diagram similar to figure 2 and sending it back to me via ipilot email! Good luck, and here is the question: You and I are flying again in a general aviation airplane during the daytime. Our properly set and accurate altimeter reads 2,000 feet. We pass over the tower that is located almost due north of the Taylor County airport. The tower symbol indicates 1,295 in bold blue letter and 270 in parenthesis. The flight visibility at this moment is 2 miles due to summer haze. Are we flying legal VFR? Email me your answers and explanations!

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About This Author:
Paul A. Craig is a Gold Seal Multiengine and Instrument Flight Instructor. He currently holds a total of 11 Flight Certificates including his ATP. Craig is a previous winner of the North Carolina and Tennessee Flight Instructor of the Year award, the NCVT Outstanding Teacher award and has served as the regional representative of the National Air and Space Museum. Craig is an FAA Aviation Safety Counselor and the author of eight books, including Pilot In Command, The Killing Zone: How and Why Pilots Die, and Controlling Pilot Error: Situational Awareness (all from McGraw Hill).
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