Ask Captain Lim ~ All about Aviation

Hi Capt Lim,

Good to have you back!!

I fly quite a bit, and enjoy taking pictures along the way. Unfortunately I usually don*t know where we are at any given time.

My camera puts a time stamp on the picture file, and since I almost always take photographs during the takeoff and landing, I know those times, and thus the time from the origination and destination airport to a given location. But the distance has me thrown. Is there a rough average speed I should consider to do this calculation?

I recently flew from Ontario (California) to Nashville (Tennessee), and we flew over a couple of airports. I knew the elapsed time, and I figured 300 mph for the speed. I used a highway map to find the approximate location, which turned out to be either Phoenix, Flagstaff or Las Vegas. Then I went to the Internet to find runway diagrams for those three cities, but none of them matched my pictures!

Aren't the air routes pretty much standardized, so if I got a set of aeronautical charts (Jeppesen?), wouldn*t they show the route the plane is taking? And isn't there a set of books that has runway diagrams?

What if I got a small handheld GPS? It would show me long/lat, but would it work inside the aircraft, and would I even be allowed to use it?

I guess I could just ask the flight attendants, but they probably have better things to do than respond to "Oh, that's cool, what is it?" and "Wow, where are we now?" kinds of questions every 20 minutes (my wife says I*m like an overgrown kid when it comes to flying)!

Larry

LARRY L. BURRISS, Ph.D., J.D.
Professor, School of Journalism
Middle Tennessee State University

Hi Larry,

In most of the modern commercial airliners (Yes, Boeing 777 included) today, your current position in flight is shown on the "moving map" (we call it the Air Show) in front of you. So, you are where you are on the map! Arrgh… maybe you were not on board one of those planes :-) !

What you did to calculate your air position were also the basic procedure used by pilots except that they knew the actual ground speed. In your case, using 300 mph, if you were on a jet plane, would give you the wrong position! You should work on 500 mph, as this speed would give you a better estimate. As a rule of thumb, this would be in the region of 8 miles per minute. Hence, in 10 minutes, you would have flown, roughly 80 miles.

Yes, air routes are pretty standardized but planes do not necessarily fly along them all the times. They may do "dog-leg" or deviate along the airways to avoid weather, for instance, the nasty thunderstorms.

Detailed runway diagrams are found in the Jepperson Charts and those from books are less accurate as they are hardly updated.

As far as the handheld GPS is concerned, it is like the PDA or Palm Top (see here). You cannot use it during the take off, approach and landing but you can do so once the seat belt signs are off (usually passing 10,000 feet) during the cruise. Ah, from what I know, a GPS receiver requires an unobstructed line to each satellite it is tracking. A reliable position indication requires signals from at least 3 satellites, and if elevation of the location is required, at least 4 satellites are necessary.

Inside the airplane cabin, it may not be possible for the satellite signals to reach the GPS unit and therefore, it is unlikely that you can use the GPS on board effectively. In other words, the incoming signals for the GPS may be shielded by the airplane*s structure.

Yes, it is okay to ask the Flight Attendants about your current position during their lean period in flight. I do get such requests through the cabin crew quite often on long haul flights. It helps to keep me awake :-) !