Radar and Air Traffic Services re: Crash of Bhoja Air 213/April 20, 2012

Air Traffic Services (ATS) Analysis

To analyse the role of Air Traffic Services (ATS) in this accident, the facts pertaining to this domain were considered, by segregating them into various essential activities of ATS and determine the specific areas which could have contributed directly or indirectly towards the causation of the accident. The possible activities related to ATS could be divided into the following domains:

  • Role of ATS (Radar or Non-Radar)
  • Role of Radar in ATS (General)
  • Role of Radar in Bad Weather Conditions
  • Organizational Responsibilities

 Role of ATS (Radar / Non-Radar)

Pakistan being signatory of Chicago Convention for the provision of air traffic services follows the standard ATS rules and regulations specified by the ICAO in its Annexes and documents. The following objectives of ATS are extracted from ICAO Annex 11.

The objectives of air traffic services shall be to:

  • Prevent collisions between aircraft
  • Prevent collisions between aircraft on the manoeuvring area and obstructions on that area
  • Expedite and maintain an orderly flow of air traffic
  • Provide advice and information useful for the safe and efficient conduct of flights
  • Notify appropriate organizations regarding aircraft in need of search and rescue aid, and assist such organizations as required.

 Air Traffic Control (ATC) Service

ATC service is provided by ground-based controllers who direct aircraft on the ground and in the air. The primary purpose of ATC systems worldwide is to:

  • separate aircraft to prevent collisions
  • to organize and expedite the flow of traffic
  • to provide information and other support for pilots for safe conduct of flights.

Preventing collisions is referred to as separations, which is a term used to prevent aircraft from coming too close to each other by use of lateral, vertical and longitudinal separation minima. ATC is providing additional services in the form of advice and information to pilots for the safe conduct of flight, it may be weather and navigation related information or any other information. The services provided to aircraft whether it is procedural, or radar based on charter given by the ICAO is uniform all over the world.

ATC service is provided throughout Pakistan in selected airspaces and ATS routes. The service is available to all users (private, military, and commercial aircraft) when flying in designated airspaces or at designated ATS routes.

When controllers are responsible for separating some or all aircraft, such airspace is called “controlled airspace” in contrast to “uncontrolled airspace” where aircraft may fly without the use of air traffic control service. Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to follow, or merely flight information (in some countries known as advisories) to assist pilots operating in the airspace. In all cases, however, the pilot in command has final responsibility for safety of the flight and may deviate from ATC instructions in emergency or to avoid severe / adverse weather areas.

Organizational Responsibilities

BBIAP Islamabad is a joint user airport being used by both Civil Aviation Authority (CAA) Pakistan and Pakistan Air Force (PAF). The approach control service is provided by CAA radar whereas the aerodrome control service is provided jointly by CAA and PAF qualified controllers under a formal letter of agreement as per ICAO requirement to avoid any confusion. The qualified CAA radar controllers are deployed for providing radar and procedure approach control service in a pre-defined area around BBIAP, Islamabad. On the day of accident, instrument approaches on both the RWY- 12 / 30 were available for landing. There are number of restricted / prohibited / danger areas around BBIAP, Islamabad, which compel controllers and pilots to remain within a specified airspace, even in severe weather scenarios.

 Role of Radar in ATS (General)

Radar controller is an air traffic controller providing air traffic control service to aircraft using surveillance facilities (radar). The purpose of using radar in the provision of ATC service is to achieve the following extra benefits about the objective of ATS:

  • Reduction in conventional separation
  • Reduction in track miles of the aircraft
  • Monitoring the phases of flight and clearances
  • Expeditious flow of air / ground movement of aircraft
  • Provision of navigational assistance
  • Provision of weather related information

All over the world, ATC radars receive echoes of higher moisture contents in atmosphere, in the form of clutters. Although, technically, it is possible that an echo could be associated with birds, volcanic ash, etc. Even then controllers tell pilots the location of significant areas of moisture in the form of clutter on radar scope when it appears that it may affect the aircraft’s flight path.

Controllers also aid in the form of course deviations when requested by the pilot. Although the weather picture presented on ATS surveillance radar is always not very accurate as these radars are not meant for weather purposes even then the information is shared with pilots just to caution the pilot, otherwise the weather radar onboard the aircraft is more accurate / sophisticated and can scan the predicted route / flight path of the aircraft. Hence pilots based on weather radar picture available to them, select and fly the safest possible tracks in coordination with ATC to avoid significant weather prevalent areas.

The radar controller on duty provided standard ATC service to ill-fated Flight BHO-213 as per ICAO documents Doc 4444, Annex 11 and directory of duties. There was no breach of procedures observed during investigations from ATS point of view. Aerodrome information including weather information was being broadcast through ATIS and it was observed that as per the CVR transcript cockpit crew of Flight BHO-213 never selected and monitored the ATIS being broadcasted by BBIAP, Islamabad, however ATIS AIIAP, Lahore was monitored by the cockpit crew during flight. The radar controller owing to inclement weather conditions around BBIAP, Islamabad guided the aircraft based on weather picture available to him on the radar scope which was also appreciated by the Captain of aircraft during flight. There was no contrast or deviation to international standards which was observed by the investigation team.

 Role of Radar in Bad Weather Conditions

 ICAO specifies in its documents that Controllers are required to provide the most appropriate advice / information to pilots of an aircraft requesting navigational assistance when avoiding areas of adverse weather conditions. The following are the guide lines which are to be followed in weather avoidance scenarios:

 Radar Data

Radar echoes of weather system depict frontal weather activity that generated severe turbulence and caused high wind shears that were responsible for the microburst. Radar echoes from 1820 PST to 1845 PST of 20th April 2012 inserted above depicted that weather activity over BBIAP and east of Islamabad & Kashmir was more severe and

 Information regarding Adverse Weather

Information should be issued to the affected aircraft in sufficient time to permit the pilot to decide on an appropriate course of action when an aircraft appears to penetrate in an area of adverse weather, including that of requesting advice on how best to circumnavigate the adverse weather area, if so desired.

 Note: Depending on the capabilities of the ATS surveillance system, areas of adverse weather may not be presented on the situation display. An aircraft’s weather radar will normally provide better detection and definition of adverse weather than radar sensors in use by ATS.

When vectoring an aircraft for circumnavigating any area of adverse weather, the controller should ascertain that the aircraft can be returned to its intended or assigned flight path within the coverage of the ATS surveillance system and, if this does not appear possible, inform the pilot of the circumstances.

 Note: Attention must be given to the fact that under certain circumstances the most active area of adverse weather may not be displayed.

The cockpit crew should notify the concerned ATS unit and request clearance to deviate from track, advising, when possible, the extent of the deviation expected, expressed in new heading and for how long the cockpit crew intends to proceed on the deviation. When the pilot initiates communications with ATC, a rapid response may be obtained by stating “WEATHER DEVIATION REQUIRED” to indicate that priority is desired on the frequency and for ATS response. When necessary, the pilot should initiate the communications using the urgency call “PAN PAN” (preferably spoken three times). The pilot shall inform ATC when weather deviation is no longer required, or when a weather deviation has been completed and the aircraft has returned to its cleared route.

To achieve the basic objectives of ATS (Radar), radar controller requires Position indications (tracks) and map on the Radar scope. The weather information available to radar controller on the radar scope is additional information to caution the aircraft about the weather phenomena in an area, otherwise the objectives of ATS do not hold controller responsible for separating aircraft from severe weather activity at any phase of flight as stated in above paragraphs, aircraft are advised / cautioned on the basis of weather information if available on radar and the depicted picture of weather is passed to the concerned / affected aircraft so that pilot can analyse the severity and strength of weather activity with the help of aircraft weather radar as the aircraft weather radar could provide more accurate weather picture and its strength to the cockpit crew. Pilots always select a proper and safe course of action and get necessary clearances from concerned ATS unit.

The aircraft are normally cleared on the requested tracks in weather avoidance situations or alternate safe routes are suggested by ATS units to avoid danger / restricted areas. Whenever cockpit crew in any circumstances considers the ATS cleared track / route in-appropriate, they may ask for other suitable track as the final responsibility of the safety of the flight lies with the captain of the aircraft as per ICAO Annex 2. Air traffic controllers provide flight information to aircraft; it is the pilot to take best possible decision in the interest of safety of the aircraft based on information provided to him, however, by international rules and regulations controllers are not involved in decision making process whether to continue for destination or divert to alternate airport.

The Flight BHO-213 was the only aircraft in the area, so the full attention was given to the aircraft and radar controller remained in touch with the aircraft till last moment. The Flight BHO-213 was provided the available weather information / briefing. The captain of Flight BHO-213 at one stage appreciated the controller, upon passing useful weather briefing to him.

The Captain requested radar controller to circumnavigate through the narrow gap of CB cells, whereas radar controller was not picking up any active weather cells on radar scope between radial 1600 to 2200. To clear the confusion and be sure of the actual prevalent weather conditions between the above-mentioned radials, the radar controller asked the cockpit crew to confirm any weather activity towards southeast of Islamabad at about 10 to 15 nm. Radar controller on confirmation from the Captain that southeast of Islamabad is clear of weather till 40 nm, provided radar vectors to carry out an ILS approach RWY 30. The aircraft was never circumnavigated around CB cells by the radar controller. Moreover, the Captain after turning onto heading 360 again confirmed that the area ahead of the aircraft is clear of weather and the actual weather is in the north of BBIAP, Islamabad.

The cockpit crew of Flight BHO-213 may have used their best judgment based on information provided by the radar controller and the onboard weather picture available to them. Moreover, cockpit crew may have used the option of discontinuing on the heading advised by the radar controller, if it was not clear of weather for the safe conduct of flight. The detailed analysis on cockpit crew mutual discussions on weather en-route to BBIAP, Islamabad and their handling of situation is given in Operational Analysis part of the investigation. Radar controller did not have any information on the presence of wind shear near BBIAP, Islamabad during the final approach of ill-fated Bhoja Air aircraft as per the available weather information.

 Meteorological information.

On 20th April 2012 the weather around OPRN was forecasted to be cloudy with chances of thunderstorm and rain. The same was passed in advance through TAFs. Duty Met Officer issued weather warning for thunderstorm rain at 1430 hrs on 20th April 2012 for OPRN which was initially valid from 1500 till 1800 hrs and later was extended. It included 1-2/8 CB at or above 2500 feet AGL with reduction trend of surface visibility from 3-1 km or even less in precipitation and wind 20-40 knots gusting 65 knots or more for BBIAP Islamabad and 50 km around. The detailed Meteorological analysis is given in subsequent paragraphs.

 Boeing Atmospheric Analysis

A weather analysis was conducted by the Atmospheric Physics Group at Boeing USA to determine whether the conditions were conducive to generating downdrafts of the magnitude observed in the calculated data. After an analysis of infrared satellite imagery, surface observations, and model derived thermodynamic profiles, it was concluded that high-based thunderstorm activity was present in Islamabad at the time of the accident.

Surface observations showed thunderstorms with gusty and variable winds associated with the convective system. Wind speeds between 20 and 34 knots were reported while the convective system was in the vicinity. Also, a theoretical downdraft strength potential of 40-50 knots (68-84 fps) was derived from the thermodynamic profile. Given these facts, the accident occurred in an environment with a high probability of producing strong downdrafts.

Weather Analyses

Infrared Satellite Imagery. At 13:30 UTC on 20 April 2012 the Meteosat infrared satellite image showed following picture of the regional area.

thumbnail_IMG_3418

According to the above mentioned weather picture, BBIAP Islamabad (OPRN) was covered by a large canopy of bright white (cold topped) clouds.

An in-depth study was conducted by the meteorological investigation team member to find out all the information related to the reported / prevalent weather conditions in and around BBIAP, Islamabad prior to and at the time of accident. The relevant METARs, satellite picture of Pakistan weather, the weather relayed through ATIS and the weather announced by Approach Radar, along with weather analysis by Boeing by utilizing all available sources to ascertain the exact weather parameters on the mishap day were studied, the details are appended below:

As the CVR recording revealed exposure of mishap flight to wind shear, therefore, this specific phenomenon was studied in detail to see its contribution towards causation of occurrence.

 Wind Shear. Wind shear is a micro scale meteorological phenomenon in which sudden and drastic changes in wind direction and speed take place with altitude over a short distance. It is usually associated with a microburst that often occurs in the vicinity of thunderstorms resulting in conditions that can cause rapid changes in lift and hence the attitude / altitude of the aircraft.

Generally, the winds travel horizontally, but under certain conditions in thunderstorms and frontal system, wind shear will travel in a vertical direction, causing up and downdrafts. 

thumbnail_IMG_3413

Microburst

Microburst (Downburst) is the most violent form of downdraft from a thunderstorm. It is characterized by an intense and localized descent of cool air, causing a sudden outflow of horizontal winds above the ground with a typical horizontal extent of a few kilometres; the rapid down-rush of air in the downdraft is caused by hail storm or heavy rain. As the pocket of cooler air hits the ground, it spreads out in all directions.

thumbnail_IMG_3419

 

thumbnail_IMG_3420

Microburst wind shear is an extremely violent downward blast of air that hits the ground and radiates outward with its sharp shifts in wind speed. An aircraft flying through a microburst may first encounter an increasing headwind and lift, then a downdraft from above the aircraft, followed by an increasing tailwind and sink. To overcome the adverse effect of the microburst, the pilot needs to take timely corrective actions as recommended by the manufacturer to ensure aircraft safety.

Meteorological Analysis

On 20 April 2012 the weather around OPRN was forecasted to be cloudy with chances of thunderstorms and rain. The same was passed in advance through TAFs. Duty Met Officer issued weather warning for thunderstorm rain at 1430 hrs on 20 April 2012 for OPRN which was initially valid from 1500 hrs till 1800hrs and subsequently extended. It included 1-2/8 CB at or above 2500 feet AGL with reduction trend of surface visibility from 3-1 km or even less in precipitation and wind to be 20-40 kts QNT 65 kts or more for BBIAP Islamabad and 50 km around.

 METARs revealed that the destination (OPRN) had a thunderstorm warning with 1/8 TCU and alternate AIIAP, Lahore (OPLA) had DRW warning with no sig clouds at the time of departure of Bhoja Air. All the Met reports in the shape of TAFs, Mets and Species were timely delivered by the concerned departments. The existing TCU at destination was converted into CB at 1251 hrs and was promptly reported by Duty Forecasting Officer but by then the ill-fated aircraft was already en-route to OPRN. The comparison of three hours MET reports of Islamabad is given below:

 Time in UTC

Weather Report

  • 1100 SE 16KTS VIS 6KM HAZE 1TCU030 4SCCU040 4AC100 QNH 1009 TEMP 32/13 WEATHER WNG FOR TSR valid up to 1300.
  • 1200 SE 22KTS VIS 6KM HAZE 1TCU030 4SCCU040 6AC100 QNH 1008 TEMP 31/12 WEATHER WNG FOR TSR valid up to 1300.
  • 1300 SW 20KTS VIS 4KM TS 1CB025 4SCCU040 6AC100 QNH 1009 TEMP 25/15 WEATHER WNG FOR TSR valid up to 1600.

As per the flight plan of Flight BHO-213 Lahore was kept as alternate aerodrome, in case pilots decide to divert due to unfavourable landing conditions at destination they might go to the alternate aerodrome submitted in the flight plan. The weather situation at alternate aerodromes was as per following detail.

Satellite products

  • Satellite imageries also depicted similar pattern of weather from 0900UTC when the severe weather development started over central KPK. The system first migrated slowly towards east and then northeast.
  • At 1301UTC, the core was southwest of BBIAP, Islamabad whereas at 1401UTC, it engulfed the entire region of Islamabad-Rawalpindi, Mangla (Jhelum) and up north.
  • To further focus into the weather system, infrared imageries received at 1315UTC and 1345UTC were also analysed in detail. Satellite data depicted that severe high impact weather activity was approaching airfield at 1315 UTC and engulfed en-route & BBIAP between 1315-1345 UTC.

 Radar Data

thumbnail_IMG_3412

  • Radar echoes of weather system depict frontal weather activity that generated severe turbulence and caused high wind shears that were responsible for the microburst.
  • Radar echoes from 1820 PST to 1845 PST of 20th April 2012 inserted above depicted that weather activity over BBIAP and east of Islamabad & Kashmir was more severe and embedded Cb clouds producing updrafts and downdrafts were mapped by radar system during the period.

 Weather Related Organizational Responsibilities

  • In fact, all the set procedures were adopted by various stake holders. Met Flight folder was issued to Flight BHO-213 from Met Office Karachi that contained METARs, TAFs, Warnings, Satellite Imagery, significant weather chart and wind charts. Significant weather chart, METARs, TAFs etc were a meaningful source of information to caution the pilots about disturbed weather in northern parts of country (Lahore-Islamabad).
  • Weather updates were continuously provided to Islamabad Approach Radar Controller by Meteorological Department BBIAP, Islamabad and the same was communicated by Islamabad Approach Radar Controller to Flight BHO-213 when it contacted for the purpose. In fact, at 1327UTC, Flight BHO- 213 appreciated the radar controller for weather observation provided to him by saying “it was very nice whatever you told me”.

 The weather observation reports on 20 April, 2012 before and after the accident are as follows:

Time UTC –BBIAP Weather Report

  • 1100 SE 16KTS VIS 6KM HAZE 1TCU030 4SCCU040 4AC100 QNH 1009 TEMP 32/13 WEATHER WNG FOR TSR valid up to 1300.
  • 1200 SE 22KTS VIS 6KM HAZE 1TCU030 4SCCU040 6AC100 QNH 1008 TEMP 31/12 WEATHER WNG FOR TSR valid up to 1300.
  • 1300 SW 20KTS VIS 4KM TS 1CB025 4SCCU040 6AC100 QNH 1009 TEMP 25/15 WEATHER WNG FOR TSR valid up to 1600.

Lahore Weather Report

  • 1100 SW 230/17G28KTS VIS 3500M DRDU SCT040 SCT100 QNH 1009 TEMP 32/13 TEMPO 22030KTS 2000.
  • 1200 SW 240/18G28KTS VIS 3500M DRDU SCT040 BKN100 QNH 1009 TEMP 30/12 TEMPO 22040KTS 2000.
  • 1300 SW 230/13KTS VIS 4000M HZ SCT040 BKN100 QNH 1010 TEMP 27/12 TEMPO 22030KTS 2000 DRDU.

 Peshawar Weather Report

  • 1100 SW 20KTS VIS 6KM HAZE STRA FEW030CB SCT040 BKN100 QNH 1012 TEMP 23/17
  • 1200 SW 12KTS VIS 6KM HAZE STRA FEW030CB SCT040 BKN100 QNH 1011 TEMP 21/18
  • 1300 SW 16KTS VIS 4KM HAZE STRA FEW030CB SCT040 BKN100 QNH 1011 TEMP 21/18

The following weather warnings were issued for Islamabad region on the day of accident:

 Time in UTC –Weather Warning

1200 (METAR)

  • WX WNG FOR TSRA OVER OPRN AND 50KM AROUND DURING THE PERIOD OF 20-1000Z UPTO 20-1300Z SURFACE WIND NE-NW 20-40KT QNT 65KT OR MORE SURFACE VISIBILITY 3 TO 1 KM OR LESS DUE TO PPTN MORDERATE / SEVERE TURBULENCE MAY ACCURE 1-2/8 CB AT 2500FT ABOVE GROUND LEVEL PROB 70%)

1300 (METAR)

  • (OUR PREVIOUS WX-WNG FOR TSRA OVER OPRN AND 50KM AROUND IS FURTHER EXTENDED UPTO 20-1600Z. S/WIND NW-NE 20-40KT QNT 65KT OR MORE. S/VIS MAY REDUCE 3-1KM OR LESS IN PPTN MOD/SEV TURB MAY OCCURE IN 1-2/8CB AT 2500FT AGL PROB 70%=)

1400 (METAR)

  • WX WNG FOR TSRA OVER OPRN AND 50KM AROUND DURING THE PERIOD OF 20-1600Z TO 20-1900Z=

Summary

  • Satellite image analysis and its animation depicted that Flight BHO-213 encountered severe weather system (high impact).
  • NWP products fairly depicted that severe wind shear caused severe thunderstorms resulting in updrafts and downdrafts. NWP atmospheric sounding depicted severe convective cloud environment which is characterized by a dry sub-cloud layer (i.e. high convection near the surface).
  • The surface inflow to the convective system had a temperature / dew point spread of 350 Fahrenheit (90F/55F), which indicates potential for strong convective downdrafts. Also, its Downdraft Convective Available Potential Energy (DCAPE) value is more than 300 J/kg. Such a high DCAPE has the potential to produce a downdraft of more than 40 knots. Therefore, sounding of Islamabad confirms the development of Cb clouds and thunderstorms activity with vertical winds more than 40 knots.
  • The surface observation reports before and at the time of Bhoja Air aircraft accident near BBIAP, Islamabad confirmed high impact weather activity (TSRA, surface winds 36 knots at 1318 UTC and TSRA, surface winds 24 knots at 1340 UTC).
  • Based on above facts, it is concluded that this accident occurred in an environment with a high probability of producing strong downdrafts coupled with strong wind shear.

 Courtesy and source: Safety Investigation Board Report, Civil Aviation Authority, Pakistan 

 

 

 

 

 

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