Expert: Ned Dolan - 11/25/2009

Question
QUESTION: Are pilots of commercial aircraft such as American, Continental, Delta, Northwest, United, US Airways, and TWA required to monitor 121.5 MHz at all times in the United States?  

Is it difficult for a boat (watercraft) of able to establish contact in case of a mayday with a commercial aircraft (example Delta) and or air traffic control, if they transit a mayday on 121.5 MHz, yes or no?  

Are they allowed to do this, yes or no?  The boat is unable to reach any body on Marine VHF radio Channel 16, (156.8 MHz).  

Is this correct, yes or no?  

The aircraft emergency frequency (also known as guard) is a frequency used on the aircraft radio band reserved for emergency communications for aircraft in distress. The frequencies are 121.5 MHz for civilian, also known as International Air Distress (IAD) and 243.0 MHz for military use, also known as Military Air Distress (MAD). Both are in use at the international level.  In the United States, 121.5 MHz is monitored by most air traffic control towers, FSS services.  

If an aircraft violates or is on a trajectory that will violate Restricted or Prohibited airspace, it will be warned of military interception on 121.5 MHz. The frequency may also be used by ATC to establish contact with an aircraft that has inadvertently switched to an incorrect frequency.  Pilots are strongly recommended to monitor 121.5 MHz at all times.

ANSWER: In my experience commercial airline pilots do not, and are not required to,  monitor 121.5 during most domestic operations. They do monitor 121.5 when operating over oceanic routes. I am not sure if this is a requirement or just normal practice.

Therefore is would be easy for a watercraft to establish contact when within range if the aircraft is on an oceanic route.

I have no idea of whether they are allowed to do this or not. In an emergency I don't think anyone would care about the legalities of such a contact.

Your last two paragraphs sound likely, but I don't have the knowledge to vouch for the wording of each.

Hope this helps.    Ned





---------- FOLLOW-UP ----------

QUESTION: How close does a boat have to be to a commercial airliner to establish radio contact on 121.5 MHz in miles? (Example one mile or 100 miles)?  Would a commercial airliner inform air traffic control if they receive a distress call on 121.5 MHz  but the sender only able to transmit not receive radio contact, on a over oceanic routes (example Atlantic Pacific ocean)

ANSWER: Since VHF is line of sight, a normal maximum distance for the type of radios we have on board commercial aircraft would be about 250 - 300 miles depending on altitude. I don't know how powerful the shipboard transmitters are, but your receivers should be hearing us at those distances. Ned

---------- FOLLOW-UP ----------

QUESTION: From how can an aircraft receive a message with a 5 watt Compact Air Band Transceiver on 121.5 MHz, which is in a boat in the ocean (Example one mile or 100 miles)?  
Generally aircraft at 33,000 feet should be able to receive that signal father than another boat in a boat in the ocean.  Is that correct?  

Answer
VHF transmissions are received in "line of sight". That means if the receiver is within line of sight the transmissions should be received.

As to how far a 5 watt transmitter can send a signal I can't answer that question. It should be directed to a radio technician.