FLARM Briefing


The See and Avoid principles, involving the development and continuous use of sound LOOKOUT and SCANNING techniques by glider pilots is the primary means of COLLISION AVOIDANCE.

The development of sound Situational Awareness by pilot's is another good aid to collision avoidance. Situational Awareness is built using all available information sources, including LOOKOUT, the radio and (where fitted), FLARM.

FLARM should not be relied upon as a primary means of COLLISION AVOIDANCE due to the fact that,

a) Not all gliders are fitted with FLARM.

b) Few powered aircraft are fitted with FLARM.

c) FLARM may suffer interference, ie from Mobile Phone.

d) FLARM units may fail (flat batteries, fault, not turned "ON", etc).

e) There is no substitute for a good LOOKOUT.

There is nothing to take the place of a high level of LOOKOUT and appropriate SCANNING techniques.

FLARM operates by transmitting position information and receiving position information from other FLARM units. The low power of the transmitted signal results in a relatively short range.

Operating FLARM

It is important to understand the lights (LED's) in the FLARM display.

When turned "ON", the unit proceeds through a start-up sequence,

  • Every LED flashes Green.
  • The "Alarm" sounds briefly.
  • Every LED flashes RED.

After this test sequence, the Power LED will continue to flash green, to indicate power is ON. Should this LED change to a flashing RED, it indicates the supply voltage is outside normal limits (high or low voltage).

The GPS LED will remain a steady RED until a GPS signal is received, when the LED will turn OFF. (If this LED remain illuminated, a GPS signal is not being received and position information cannot be transmitted.

The Tx/Rx LED will remain a steady RED until the FLARM unit begins transmitting its position data. Should this LED be steady GREEN, it indicates it is not receiving any other FLARM unit but is ready for operation.

There are two (2) other "sets" of LED's,

a) A circle of 9 LEDs (inc one in the centre). These indicate the approximate position of "conflicting" aircraft – left or right, in front or behind. The LED in the centre indicates conflict directly above or below.

b) A vertical row of lights which indicate

- above, > 200 metres
- same level, +/- 200 metres
- below, > 200 metres.

Interpreting FLARM Indications

It is now obvious that the FLARM units LEDs are able to change colour (RED or Green). The LED's will operate together with an audible alarm depending upon the forecast Time To Impact (TTI), ie

First alarm level, 18 – 20 seconds TTI
Second alarm level, 13 – 15 seconds TTI
Third alarm level, 8 – 10 seconds (TTI)

The alarm level will persist as long as the alarm level is appropriate.


TTI Alarm Level  LED/Audible Alarm
> 20 secs None None
18 – 20 secs 1 Steady RED LED.
13 – 15 secs 2 Steady RED LED, Audible warning.
8 – 10 secs 3 Flashing RED LED, Audible warning.


When SCANNING the field of view, include the FLARM unit in the scan - the earlier you are aware of another glider / aircraft, the earlier avoiding action may be initiated.

FLARM operation may be adversely impacted by other electronic apparatus, including mobile phones, consequently, pilots are advised to turn these OFF if possible while flying.


John Hudson

Last update 10/03/08