Expert: Marc Ebelini - 8/15/2010

Question
Dear Sir,
I am an A & C apprentice, my question is
What is a typical range for VHF transreceiver
when we say LOS?
Inspite of all the advancement in avionics why do
present day aircrafts (eg. the B-777) use the
traditional compass and ADF in the cockpit?

Answer
The typical range of VHF is 25 miles depending on the terrain. Naturally aircraft range is increase depending on he altitude. Amateur radio operators (Hams) can usually increase the land range by increasing the power and  using special antennas.
The compass is a requirement for all aircraft especially commercial aircraft. I once grounded a Convair because regular screws were used to replace the damaged non magnetic ones. Because this would require the aircraft to have the compass reswung and a new compass correct card installed before the next flight.
Depending where the aircraft is flying. Many places do not have good navigation coverage for various reason. So homing on a broadcast radio signal may be required. Back to the compass. when you lose all electrical power the compass still works. And that is your only hope.
Please do not confuse the flux gate compass with the plain magnetic one. And never assume the runway number is the correct magnetic heading. To many factors are involved to discuss here. Hope I have answered your question to your satisfaction.