Bicester Aviation Services
IGC Approved Secure Flight Recorders
This Page - Introduction to SFRs
SFR 5 - Cambridge 302/302A and LX7000 IGC Pro
SFR 6 - Comparison of current SFRs
Following the introduction of GPS, over the last few years rapid progress has been made in the field of GPS flight recording. This is particularly significant in the gliding field where GPS flight recorders (FR) have replaced cameras and barographs as evidence for turning point verification.
This series of articles traces the development of flight recorders for gliding and then goes on to discuss individual models in detail. The overall aim is to provide a sound basis to enable a deeper understanding of the principles involved and give some help in choosing a flight recorder.
The FAI Flight Recorder Standard
The first GPS FR was the RD/Skyforce logger which recorded GPS position and GPS altitude every 30 seconds. After flight the record could be down loaded and the flight examined in detail. This simple recorder had no built in security features and the data could easily be altered. In 1994 the International Gliding Commission (IGC) of the Federation Aeronautique Internationale (FAI) asked their GPS sub-committee to examine the problem and recommend a way ahead. The sub-committees first actions were to lay down standards of hardware that manufacturers of FR were required to meet in order in order to ensure a good level of data security, and to formulate a common data file standard (.IGC) so that different manufacturers FR were capable of being analysed by the same computer program.
IGC then approved the formation of a small sub committee of IGC, titled the GNSS Flight Recorder Approval Committee (GFAC), who were tasked by the IGC to test and evaluate different FR, and to recommend to the IGC that they approve those models that were found to comply with the IGC criteria. This committee has currently approved over 50 recorders from 17 different manufacturers.
All these recorders produce a secure .IGC computer file in a standardised format that can be be transferred to a floppy disc and subsequently analysed to prove that the glider pilot completed the designated task. The .IGC file is in ASCI format, which is computer jargon for 'plain text', and all approved recorders, whatever their make, can produce an .IGC flight file in this common standardised format. One of the most important criteria was that the file should be secure and at first sight an ASCI text file looks anything but secure. Security is achieved by use of a clever algorithm that when the flight data is transferred from the recorder, compiles a complex checksum which is written into the last field of the .IGC file, and which is able to detect if any of the flight data has been subsequently manipulated. This verification procedure can be carried out at any time by simply using a computer to examine the .IGC file using a copy of the recorder manufacturers validation file.
Some recorders have the capability of recording "noise" using an internal microphone so that the engine noise of self sustaining (turbo) gliders and motor gliders can be ascertained. This feature is discussed later.
Types of Secure Flight Recorders
There are four basic groups of FR available to glider pilots, each with different capabilities. Three of these groups are FAI/IGC approved recorders while the fourth group have no formal approval. In order to help identification of the approval status of a particular FR, they have been divided by IGC into 3 groups, All Flights, All Badges and Diamond Level.
An integral part of FAI approval is a detailed FAI document that is issued for each approved recorder. The document is informative giving the main attributes of each recorder together with helpful notes for both pilots and official observers. Copies of these approval documents can be seen on the FAI Web Site and downloaded as required. The recorders mentioned below are restricted to those which are common in the UK; there are number of others which are not imported into the UK
"All Badges" IGC Approved Flight Recorders
This group contains those units that have been approved under the "All Flights" category, normally for reasons of reduced security integrity. They can be used to validate all flights with the exception of World Records. The recorders currently in this group include the;
"Diamonds Level" IGC Approved Flight Recorders
In the main, the units in this category are Flarm related products together with other models that have been downgraded for reasons of reduced security. They can then be used as evidence for FAI Silver, Gold and Diamond badges and UK competitions.
Unapproved Flight Recorders
These recorders are not IGC approved. They include recorders that have
never had IGC approval, together with others that have been downgraded for lack
of security. Some National Aero Clubs (NAC) have
decided that some recorders in this group can be used under local control.
Recorders in this group are:
GPS Map Datums
One of the mandatory requirements for a FR to receive IGC approval is that it is fixed on the WGS84 map datum. Briefly, map datums are assumptions made about the actual shape of the earth when early mapmakers transferred the earth's oblate spheroid shape onto a flat sheet of paper. In order that a GPS can give a correct position on a particular map, the GPS map datum and the datum used in the map construction have to be the same. There is at last some standardization in datums and the preferred setting is World Geodetic Survey 1984 (WGS84). Accordingly, this setting is required to be permanently fixed in the GPS engines of all "All Flights" recorders.
The map datum of an Off the Shelf (OTS) GPS unit is user selectable. However, the selected datum only appears in the NMEA output of certain Garmin GPS units. It is for this reason that the EW Models A, B and D recorders can only be used with certain specified Garmin models. All GPS engines output positional data in a ASCI (plain text) stream of characters. This ouput stream conforms to an original nautical standard derived by the National Maritime Electronics Authority, or NMEA, and is the basis of the data that is recorded in the FR. Normally, the NMEA data output does not contain details of the map datum that is set in the unit; except for the later Garmin units. Therefore, when an approved Garmin unit is connected to an EW, the map datum set on the GPS is stored in the secure flight record and can be subsequently verified.
Most IGC FRs have some method of recording engine running in a motor glider or
turbo. The usual arrangement is to have an integral microphone inside
the unit, the output of which is filtered, amplified and then recorded in the FRs memory. The arrangement works well and the engine running signature is
easily recognisable on the subsequent analysis. A print of the output of the
Engine Noise Level (ENL) is shown below. This recording came from a turbo
and the noise signatures of the tug during the launch, the retraction and
extension of the landing gear, the running of the turbo engine and the landing
can all be identified. The ENL system requires no action on the part of the
pilot; does not require specific mounting arrangements; is active immediately
the flight starts; and any engine running during flight is easily identified.
Other systems used to detect engine operation are a vibration recorder which also works well. However, such a system requires that the FR be mounted in direct contact with the glider structure; if the FR is wrapped in a sweater and then stowed in the baggage compartment, any subsequent engine operation can be difficult to analyse.
Other FRs use an external switch which detects when the engine doors are opened, the pylon raised or the ignition switch operated. Such systems rely on external wiring from the FR to the sensor or switch and such wiring needs to be checked by an Official Observer (OO) both before and after flight if the FR is going to used for flight evidence. This can sometimes be difficult to arrange as an OO is not always present before launch. One FR uses a slight variation of this system in that it measures an electrical voltage which is proportional to engine RPM coming from the Ilec engine control unit used with Schempp Hirth gliders.
When a different GPS is being used for task navigation, it is possible that there will be a small positional difference between the co-ordinates being shown on the nav GPS when compared with those of the FR GPS. This difference, which can amount to tens of metres, is caused by each GPS engine navigating by using a different mix of satellites or constellation. If the turn point sector is only penetrated by a few metres as recorded by the nav GPS, then it possible that the FR GPS may not actually record a fix in the zone. The only sure way to combat this potential problem is to ensure that the FR actually indicates a fix in the TP zone. Most FR manufacturers have solved the problem by using the same GPS engine to provide positional information for both recorder and the navigation system.
Memory Capacity and Fast Fixing
It is very difficult to compare the memory capacity of individual FRs unless each is quoted using the same parameters. Later in this series, the memory capacity of each FR is quoted in hours and assumes that logging is being carried out at 10 second intervals. Use of the PEV (pilot event marker) or fast fix capability causes the record of fix recording to speed up considerably for a pre-determined period of time. Excessive use of the PEV facility can cause memory to be used up more quickly.
Memory Roll Over
FRs have a finite memory capacity and it is important that should the memory become full, say by excessive use of the fast fixing facility, then in most FRs the memory 'rolls over' and the oldest information is deleted to make room for current fixes rather than the unit simply stop recording. Usually, the old information relates to previous flights and it is therefore not critical when it is erased. However, at least one FR, the Volkslogger, will stop recording when the memory is full.
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