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Barometric Pressure Setting Advisory Tool (BAT)
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The Barometric Pressure Setting Advisory Tool is designed to provide an advisory warning to Terminal Control Approach controllers where there is a significant difference (greater than 6 hPa) from the London QNH. The BAT tool applies only to arrival aircraft at or below the transition altitude and works with both multi-radar tracking (MRT) and single radar information source.
A study conducted by NATS of aircraft flying within the London TMA has highlighted some operator anomalies in the setting of flight deck BPS. It was established that a clear majority of flight crews change altimeter settings in a timely manner in accordance with ICAO Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS). However, there were recorded a few examples of errant behaviour. The risk of level deviations caused by altimeter setting errors is highest when the atmospheric pressure is low and the risk is significant when the pressure in the London TMA is forecast to be less than 996 hPa.
Mode S Enhanced Surveillance (EHS) transponders may automatically provide particular flight deck parameters to air traffic systems that have not been used previously by ATC. The Mode S EHS technology makes it possible for aircraft to downlink the flight deck altimeter setting in the form of a Mode S parameter known as Barometric Pressure Setting (BPS).
Mode S BPS data is provided by a large proportion of flights in European terminal airspaces and the NATS study has shown this data to be of a very high integrity in the majority of cases. There are, however, some known problems with data supplied by some aircraft when above the transition altitude, which precludes its use by ATC. Nevertheless, although the provision of Mode S BPS is not mandated, NATS considers that the downlinked data is of sufficient integrity to be used by ATC for aircraft descending below the transition altitude and that this will contribute to the prevention of level busts by inbound aircraft.
Barometric Pressure Setting Advisory Tool Description
The tool is designed to follow these principles of operation:
- A route filter is applied such that only arrivals to specific airports are eligible. For Phase One these filters are for Heathrow, Northolt, London City & Biggin Hill.
- The Mode S Selected Altitude (SFL) and the current level (Mode C) are compared to ensure that the arrival aircraft intends to remain below the Transition Altitude.
- Mode S BPS is down-linked from aircraft. The BPS must be 6mb or more different to the London QNH and SFL data must be available
- Other criteria are applied to give flight crews the maximum amount of time to change altimeter settings without unduly risking a nuisance potential level bust indication.
The tool is NOT intended to:
- Alert on departure aircraft (apart from positioning flights that stay below the Transition Altitude). This is due to issues with incorrect data provided by certain aircraft - the last selected QNH rather than standard pressure is downlinked when operating above the Transition Altitude.
- Alert above the transition altitude (e.g. aircraft entering a terminal hold).
- Support aircraft that do not provide both Mode S SFL and Mode S BPS (approximately 80% of aircraft in the TMA provide both types of data).
- Provide an advisory to TMA positions.
The BAT advisory comprises a two tone yellow pulse of the level field (as displayed below). The BAT advisory is not acknowledged and does not appear in the Vertical Stack List (VSL). STCA takes precedence over the BAT advisory and the actual BPS value downlinked is displayed in the Mode S Data window. In the radar image below the Mode A code is displayed on line one of the track label and is usually converted to the aircraft callsign on the controller’s radar display. Line two contains the current level, the destination code and the selected altitude. The BPS advisory is shown on the target with the Mode A code of 7735, the level field will pulse yellow on the ATC display. The Mode S data window shows the actual BPS being down-linked.
Responsibilities for use of the tool
- The response to a BAT advisory is not a mandatory task for controllers, although it is encouraged for early identification of possible level busts.
- The use of BAT is not a substitute for RT read-back, which remains a mandatory controller task.
- Where a BAT advisory indicates a variation with the QNH provided by ATC, controllers must not state the incorrect QNH value which can be observed in the Mode S Data Window on the radar display.
- Controllers to complete a STAR report for any BAT advisory / resolution where safety may have been compromised but use a comments folder for all other occasions where BAT was used.
Where controllers choose to query the discrepancy, the phraseology to be used is:
“(Callsign), Check Altimeter Setting, QNH xxx” - where xxx is the local QNH being utilised
Notwithstanding the above, if an aircraft is in conflict with another, the priority is to resolve the confliction. If appropriate, avoiding action should be issued prior to confirming the correct QNH. Where sufficient time exists, the use of the following phraseology may be used to query the altimeter setting used by the aircrew:
“(Callsign) Report QNH”
This will enable flight crew to check selected pressure setting, irrespective of any BAT Advisory or QNH data displayed in the Mode S Data Window.
- Issue with very low provision of BPS data from all airlines passed by a specific aircraft type. Fix being discussed with operators.
- Possible that after aircrew are challenged, BAT advisory persists. ATC is not expected to repeat challenge.
- Emergency code selected (e.g. 7500) – the Track Data Block flashes whereas associated BAT advisory will pulse.
- STCA will alert and clear BAT advisory. If STCA acknowledged / finishes, BAT advisory will immediate resume.
- Will support positioning flights below Transition Altitude as long as in check area and correct destination code.
- From 25th November 2010, BPS data will be automatically down linked from all compliant aircraft to ATC workstations within the London Terminal Control Centre at Swanwick covering arrivals at Heathrow, Northolt, London City and Biggin Hill.
- From 10th March 2011 - BAT for Terminal Control Stansted and Terminal Control Luton for arrivals to Stansted and Luton will be implanted.
- Late spring 2011 – BAT will be implemented for Terminal Control Gatwick for arrivals to Gatwick and Redhill.
Safety Impact of BPS Monitoring
The UK CAA and NATS wish to highlight the positive safety impact that the downlinking of BPS will have on the prevention of Level Busts. Operators are to advise their aircrew that for LTMA arrivals that are cleared to descend below the Transition Altitude, the down linked BPS will be automatically monitored. Where the ground system indicates that the down linked BPS is outside a specified range, ATC at the London Terminal Control Centre may issue the correct QNH on more than one occasion as a reminder to the pilot.
When ATC pass a reminder of the QNH, it is anticipated that the pilot monitoring will cross check the altimeter settings.
- Level Bust in Holding Patterns
- Transition Altitude/Level
- Altimeter Pressure Setting
- Altimeter Setting Procedures
- Minimum Sector Altitude
Accidents and Incidents
Events in which the incorrect altimeter pressure setting was either a cause or contributing factor in a Level Bust or CFIT/near CFIT:
- BE20/SF34, Vicinity Stornoway UK, 2011 (LB LOS HF) (On 31 December 2011 a USAF C12 Beech King Air descended 700 feet below the cleared outbound altitude on a procedural non precision approach to Stornoway in uncontrolled airspace in IMC and also failed to fly the procedure correctly. As a result it came into conflict with a Saab 340 inbound on the same procedure. The Investigation found that the C12 crew had interpreted the QNH given by ATC as 990 hPa as 29.90 inches, the subscale setting units used in the USA. The Saab 340 pilot saw the opposite direction traffic on TCAS and descended early to increase separation.)
- AT45, vicinity Sienajoki Finland, 2007 (LOC CFIT HF) (On 1 January 2007, the crew of a ATR 42-500 carried out successive night approaches into Seinajoki Finland including three with EGPWS warnings, one near stall, and one near loss of control, all attributed to poor flight crew performance including use of the wrong barometric sub scale setting.)
- A310, vicinity Birmingham UK, 2006 (LB CFIT HF) (On 24 November 2006, an A310 descended significantly below cleared altitude during a radar vectored approach positioning, as a result of the flight crew's failure to set the QNH, which was unusually low.)
- LJ35, vicinity Masset BC Canada, 1995 (CFIT HF) (On 11 January 1995, a Learjet 35, crashed into the sea while conducting an NDB approach to Masset, British Columbia, Canada. The most probable cause was considered to be a miss-set altimeter.)
- B733, vicinity Helsinki Finland, 2008 (LB CFIT HF) (On 26 March 2008, a Ukraine International Airlines’ Boeing 737-300 being vectored by ATC to the ILS at destination Helsinki in IMC descended below its cleared altitude and came close to a telecommunications mast. ATC noticed the deviation and instructed a climb. The investigation attributed the non-compliance with the accepted descent clearance to the failure of the flight crew to operate in accordance with SOPs. The ability of ATC safety systems as installed and configured at the time of the occurrence was also noted.)
- MD11, vicinity East Midlands UK, 2005 (LB HF) (On 3 December 2005, the crew of a MD-11 freighter failed to set the (very low) QNH for a night approach, due to distraction, and as a result descended well below the cleared altitude given by ATC for the intercept heading for the ILS at Nottingham East Midlands airport, UK.)