Friday, October 2, 2009

Line of sight query (Room with a View)

Picture compliments of Air Canada's photographer
Brian Losito

I received this query from an anonymous pertaining to my line of site post. I thought it warranted a separate post.

On page 10-11 of the Air Command Weather Manual it states that the distance of the horizon for flat terrain in nautical miles from a height (h) in feet is equal to: 1.14 times the square root of h.

Anonymous. I checked my copy of the ACWM and it certainly says 1.14 times the square root of height in feet. From the Ground Up (page 210) utilizes the formula 1.23 times the square root of height in feet. Their example of 20,000 feet gives a distance of 174 nautical miles. I was told/taught line of sight for radio coverage is the same as distance seen due to the curvature of the earth. I used 1.23 * the square root of height for my ATPL (ATR) exam many moons
ago. I also used it now and again to determine when I would receive the ATIS (Automated Terminal Information System) when flying overseas. Sometimes when flying into London, Heathrow (LHR) from over the "pond" I determined at 37,000 feet I should receive the ATIS about 237 miles back. (For some reason LHR has not gone digital with their ATIS). It worked!

I surfed the web and many sites are using the formula: the square root of both 1.5 times the height in feet. The square root of 1.5 is about 1.23


Anonymous said...

Seeing that light and radio frequencies are both electromagnetic waves there should only be one formula. Maybe the ACWM is wrong.

Anonymous said...

LHR certainly provides "digital ATIS" as well as PDC clearances, has been like that for 1,5 years at least.

From the Flight Deck said...

Anon 1: I agree, maybe the Command Manual is in error. Having said that, it's still rated as the number one weather book in Canada. I must rise to the occasion and start writing in order to change that.

Anon 2: Thanks for the feedback. Just in from SFO (San Francisco). I haven't been to LHR in 2 1/2 years. Glad to hear they are with the times. FRA (Frankfurt) had the digital ATIS for several years. Hopefully, CDG (Paris) follows suite because their ATIS broadcasted altimeter setting both as QNH and QFE. I've entered a couple of flight decks where the QFE was used instead of QNH.

Anonymous said...

Yep CDG provides the same stuff nowadays, fortunately :)

Anon 2

From the Flight Deck said...

Anon 2. Thanks. Sounds like you fly for a living?