OPERATING MANUAL FLARM COLLISION WARNING UNIT
Version 4.06E
Page 11 of 12
March 28, 2009
13. Operating Limitations
FLARM is designed and built as a non-essential 'situation awareness only' unit to only support the pilot, and
cannot always provide reliable warnings. In particular, FLARM does not give any guidance on avoiding
action. Under no circumstances should a pilot or crewmember adopt different tactics or deviate from the
normal principles of safe airmanship. Even with FLARM installed, you remain responsible for flying the
aircraft and ensure the safety of passengers and other traffic. The use of FLARM is solely at the discretion of
the commander and his delegated crew member. Operation must be preceded by thorough familiarisation by
the commander or his delegated crew member with the Operating Manual.
FLARM will only give warnings of other aircraft that are likewise equipped with a compatible unit. FLARM
does
not
communicate with Mode A/C/S transponders and is not detected by ACAS/TCAS/TPAS or Air
Traffic Control. Likewise FLARM does not communicate with FIS-B, TIS-B or ADS-B.
Compatible FLARM units must be within range in order to provide a warning. The range is very much
determined by the type, installation and position of the radio antennae, plus the relative positions of the two
aircraft. Under
optimum
conditions the internal antennae can give a head-on range of up to 5 km; normally,
range is about 2 km, which is adequate for light aircraft and sailplanes. The radio signals can only be
received by
line of sight
. There is no FLARM signal between two aircraft on opposite sides of the same
mountain.
FLARM has to know its
current
position in order to operate. For this reason, FLARM will only operate in the
presence of good quality three-dimensional GPS reception. GPS reception is greatly influenced by the
installation and position of the antenna, and aircraft attitude; furthermore, it requires that the US-American
GPS-system is fully operational. This is particularly true during turns, when flying close to mountain slopes
and in areas known for poor reception. If the installation is poor the GPS signal quality may be reduced. In
particular, there can be rapid degradation of height calculations. FLARM resumes operation as soon as the
GPS reception quality is adequate.
Movements calculated by the GPS relate to a fixed system of
terrestrial
coordinates. In strong wind there
may be a substantial difference between aircraft heading and track, leading to a distortion of the threat
bearing. If the wind speed is one third of True Airspeed (TAS) and the yaw-free aircraft Heading is 90° out of
wind, then the threat indication displayed has an error of about 18°. If the wind is very strong, the Track can
deviate up to 180° from Heading. Under such circumstances and when circling, the calculation and warnings
given are unusable.
When close up, when two aircraft are at the same or similar height, or GPS reception is poor, the vertical
bearing indication is imprecise and fluctuates.
FLARM calculates the predicted flight path of the aircraft to which it is fitted for less than the next 20
seconds. This prediction is based on immediate past data, current position- and movement data, plus a
movement prediction model that is
optimised for the respective user
. This forecast is associated with a
number of uncertainties that increase with an extension of the forecast time. There is no guarantee that an
aircraft will actually follow the predicted flight path. For this reason, the warning issued will not be accurate in
all cases. In sport flying flight path predictions of more than 30 seconds are
unusable
. This is particularly true
for sailplanes and hang gliders. For this reason, the radio range is generally adequate.
Warnings are given at
very short notice
, i.e. the warning is given within a time frame of from a few seconds to
18 seconds, depending upon the closest predicted proximity, as calculated. The threat intensity (pitch of the
warning tone, LED block width, flash interval) flags up the threat (collision time point), but not the geometric
distance. FLARM only issues a warning if the calculation forecasts a
considerable
threat. For this reason, it is
usual - depending upon the mode selected - that no warning is given about the presence of other aircraft, in
spite of the fact that signals have been correctly received.
When a number of moving threats or fixed objects are within range, then FLARM gives warning
only
of the
most dangerous
in accordance with the threat calculation algorithm.
The pilot is unable to confirm receipt of
this warning, nor is he able to call for presentation of further threats. In spite of the warning issued for one
other aircraft or fixed objects, it is quite possible that there are several further aircraft or fixed objects that
represent a greater threat than that which has been signalled. When the unit simultaneously detects a threat
from moving and fixed obstacles, then the warning issued relates to the earliest likely collision.
