Use of DF or Radar Services Use of DF or Radar Services

   While radio navigation aids can be very useful in following a specific course, they do add to the pilot's workload, both mentally and manually, particularly under dire circumstances. The pilot must divide attention between control of the airplane and the operation of the navigation equipment. Under the emergency conditions of flying the airplane by reference to flight instruments, this is often a difficult task for experienced pilots.

   However, there is another option available to pilots of airplanes with or without radio navigation equipment if they have VHF radio communications capability. All pilots should be aware of the VHF Direction Finding (DF) and Radar service available all over the U.S., either through civil or military radio facilities. This service permits a control tower, FSS, or radar facility to give the pilot a heading to fly to reach a nearby airport or to an area of good weather. In this way, the pilot need only communicate and follow instructions while giving almost full attention to flying the airplane.

   To obtain assistance by radio and apply it effectively, some preparation and training is necessary. All pilots should become familiar with the appropriate emergency procedures and practice them when the situation permits. The actual use of designated emergency radio frequencies for training exercises is not permitted, but FAA facilities are often able to provide practice orientation and radar guidance procedures using their regular communications frequencies. In any case, the pilot must inform the ground station when the request is only for practice purposes.

   When a pilot is in doubt about the airplane's position, or feels apprehensive about the safety of the flight, there should be no hesitation to ask for help. That is the first means of declaring an emergency - use the radio transmitter and ask for help. If in actual distress, and help is needed immediately, the pilot should transmit the word MAYDAY several times before transmitting the emergency message. This will get immediate attention from all who hear.

   Emergency messages may be transmitted on any radio frequency; however, there is a frequency especially designated for such messages. The designated emergency VHF frequency is 121.5 MHz and is available on most radios installed in general aviation type airplanes. This is usually the best frequency on which to transmit and receive because almost all direction finding (DF) stations, radar facilities, and Flight Service Stations monitor this frequency. Regardless of which type of facility is contacted, that facility can help, even if only by alerting other facilities to the emergency.

   Since frequent communications may be necessary when using this service, it is recommended that the microphone be continually held in the hand. This will eliminate the need to take the eyes away from the flight instruments every time the microphone is removed from or replaced in its receptacle. After some practice, loosely holding the "mike" in one hand should not create any difficulty in using the flight controls. It is important though, that the "mike" button not be depressed accidentally, which would block the frequency and prevent the reception of further assistance. The initial request for assistance can be made on the regular communications frequency of the facility, or on the emergency frequency 121.5 MHz. Regardless of which frequency is used, it is essential that the pilot not change frequency unless instructed to do so by the operator or unless absolutely necessary.

   A direction finding (DF) station is a ground based receiver capable of indicating the bearing from its antenna to the transmitting airplane. There are HF, VHF, and UHF direction finding stations. However, only VHF stations are discussed in this handbook, since this is the type of radio equipment most likely to be in the airplanes flown by the average general aviation pilot.


If a pilot is unable to establish communication with a VHF/DF facility, or if there is doubt about whether this service is available at a particular station, the service may be obtained by calling any Flight Service Station or control tower. The request then will be relayed immediately to the appropriate DF facility. The pilot must remember, though, that VHF transmissions follow line of sight; therefore, the higher the altitude, the better the chance of obtaining this service. Depending on terrain conditions and altitude, DF service is effective up to a radius of 100 miles.

The VHF/DF operator on the ground can note the airplane's bearing from the facility by looking at a scope, quite similar to a radar scope. Each time the pilot transmits, it shows up on the scope as a line radiating out from the center (Fig. 13-6). From this the operator reads the course the pilot should fly (with zero wind conditions) to reach the facility. The pilot need not be concerned about wind drift since subsequent heading instructions will consider the position to which the airplane has drifted and a new course plotted.

   When DF services are requested by the pilot, the operator will ask if the airplane is in VFR or IFR weather conditions, the amount of fuel remaining, the altitude, and the heading. Also, the operator should be informed whether the pilot is instrument rated. If the airplane is in IFR weather conditions, the pilot will be informed of the minimum safe altitude and the current local altimeter setting will be provided. The pilot will then be instructed to transmit (by keying or voice) for 10 seconds.

   To get a good indication on the scope, two continuous transmissions of approximately 10 seconds each from the airplane are required. The volume of the transmission should remain as nearly constant as possible. This can be done by depressing the "mike" key for 10 seconds or by making a voice transmission, "Aaaah," followed by the airplane's call sign and the word "Over."

   When the airplane's bearing has been determined, the DF operator will specify the direction to turn and the magnetic heading to be flown. (i.e., "TURN LEFT, HEADING ZERO ONE ZERO, FOR DF GUIDANCE TO ARDMORE AIRPORT, REPORT AIRPORT IN SIGHT.") The number of times the transmissions must be made will vary, depending on frequency congestion, distance, and wind.

   Radar equipped ATC facilities also provide assistance and navigation service, provided the airplane has appropriate communications equipment, is within radar coverage, and can be radar identified.

   Primary radar relies on a signal being transmitted from the radar antenna site and for this signal to be reflected or "bounced back" from an airplane. This reflected signal is then automatically displayed as a "target" or blip on the controller's radarscope. For identification purposes, the pilot may be asked to turn the airplane in a certain direction to a specific heading. If the airplane is transponder equipped, this identifying turn is not necessary (Fig. 13-7).

   A transponder is an airborne radar beacon transmitter/receiver which automatically receives the signals from the ground based radar and selectively replies with a specific code. These replies are independent of and much stronger than the primary radar return and are displayed uniquely on the radarscope. Thus, by having the pilot set the transponder to a certain code, the radar controller can identify the airplane immediately. After the controller positively identifies the airplane on the radarscope, the pilot will be notified of the airplane's position and will then be given a heading to an airport or other specific point in the radius of coverage of the radar station.

   The procedures used in providing radar navigational guidance to pilots are very similar to those used for DF service. There are, however, certain terms in the use of transponders which the pilot must understand. When instructed to "Squawk" a certain code (number), the pilot should first ensure that the transponder is set to the specified 4 digit number and turned ON (not standby). If told to "Ident," the pilot should press the button marked as such on the transponder. When told to "Squawk MAYDAY" (the emergency position), the transponder should be set to 7700. The term "Vector" simply means the heading to fly to reach a certain location.

   Pilots should understand clearly that authorization to proceed in accordance with such radar navigational assistance does not constitute authorization for the pilot to violate Federal Aviation Regulations. The controller must be informed whether the pilot is instrument rated and whether the airplane is equipped for instrument flight.

   To avoid possible hazards resulting from being vectored into IFR conditions, a VFR pilot in difficulty should keep the controller advised of the weather conditions in which the airplane is operating and along the course ahead.

   If the airplane has already encountered IFR conditions, the controller will inform the pilot of the minimum safe altitude. If the airplane is below the minimum safe altitude and sufficiently accurate position information has been received or radar identification is established, a heading or VOR radial on which to climb to reach the minimum safe altitude will be furnished.