Safety Mat­ters

Safety Mat­ters and Safety Briefs are based on the AAIB Bul­letin and UK Air­prox Board re­ports, with ad­di­tional ma­te­rial from the US Na­tional Trans­porta­tion Safety Board

Pilot - - SAFETY MATTERS - Com­piled by Mike Jer­ram

Train­ing midair

Air­craft Type: 1 Cessna 152 2 Guim­bal Cabri G2 Date & Time: 17 Novem­ber 2017 at 1201 Com­man­der's Fly­ing Ex­pe­ri­ence: 1 CPL, 419 hours, around 400 on type Last 90 days: un­known Last 28 days: 19 hours 2 ATPL(H), 25,000+ hours, un­known on type Last 90 days: un­known Last 28 days: un­known

This was the C152 in­struc­tor’s sec­ond in­struc­tional flight of the day, to carry out best rate of climb and glide de­scent ex­er­cises with a stu­dent. Af­ter take­off from Wy­combe Air Park they climbed steadily to 2,000ft be­fore turn­ing on course to the lo­cal train­ing area north­west of the air­field and at 1150 con­firmed with Wy­combe Tower that they had left the cir­cuit area. There were no fur­ther ra­dio com­mu­ni­ca­tions from the air­craft, which reached 4,000ft then turned left onto a steady north-west­erly course and started a sus­tained de­scent.

The Cabri G2’s in­struc­tor had also com­pleted one train­ing de­tail that morn­ing, a nav­i­ga­tion ex­er­cise in the lo­cal area to the north-west of Wy­combe, route­ing via Sil­ver­stone and re­turn. The sec­ond sor­tie was to re­peat this with a dif­fer­ent stu­dent. They de­parted at 1147, climb­ing ini­tially to the south-west, be­fore turn­ing north then north-west on track to Sil­ver­stone. The in­struc­tor ad­vised Wy­combe Tower they were leav­ing the cir­cuit to the north, and then climbed to and main­tained an al­ti­tude of around 1,500 ft amsl. There were no fur­ther ra­dio com­mu­ni­ca­tions from the he­li­copter.

At 1201 the two air­craft col­lided. Al­though no-one saw the ac­tual col­li­sion, im­me­di­ately be­fore they came to­gether a wit­ness on the ground about 0.5nm to the south-west saw them in close prox­im­ity, at around 20m apart and “fly­ing fairly low [with] the plane glid­ing down slightly and the he­li­copter di­rectly un­der­neath the plane and seemed to be ris­ing un­der­neath it.” A 300m long wreck­age trail from both air­craft fell into a wooded area in the grounds of Wad­des­don Es­tate. All four of their oc­cu­pants were killed.

The C152 had been fit­ted with a transpon­der which trans­mit­ted air­craft pres­sure al­ti­tude to the near­est 100ft. This showed that it had reached a max­i­mum al­ti­tude of 4,130ft which was main­tained for thir­teen sec­onds be­fore it be­gan de­scend­ing, turned left onto ap­prox­i­mately 340°T and re­mained on this track through­out its de­scent, which con­tin­ued for the next two min­utes and seven sec­onds, at which point the recorded al­ti­tude was 1,530ft.

The Cabri G2 was fit­ted with a Garmin GTX335 Mode S transpon­der which re­sponded to radar in­ter­ro­ga­tion and pe­ri­od­i­cally trans­mit­ted ADS-B out, al­low­ing its po­si­tion to be re­ceived and recorded. Af­ter take­off it had turned right, climb­ing to ap­prox­i­mately 1,500ft, ini­tially head­ing north be­fore turn­ing onto a track of 340°T. Through­out the re­main­der of the flight, its al­ti­tude var­ied be­tween 1,380-1,555ft.

At 1159:10 the C152, with higher ground­speed, was ap­prox­i­mately 0.5nm be­hind the Cabri and 1,950ft above. A minute later it was 750ft above and 1,000ft be­hind. At 1200:53 the C152 was at 1,500ft, the he­li­copter at 1,455 ft. Ver­ti­cal and hor­i­zon­tal sepa­ra­tion re­duced pro­gres­sively to the point of the col­li­sion.

The AAIB re­port notes: ‘Air Traf­fic Ser­vices are avail­able in the area of the ac­ci­dent, but the lo­ca­tion is on the bound­ary of sev­eral dif­fer­ent providers, thus, an air­craft ma­noeu­vring on a typ­i­cal train­ing flight within the ac­ci­dent area would prob­a­bly need to keep chang­ing fre­quen­cies. Ex­ter­nal com­mu­ni­ca­tions can cre­ate an ad­di­tional dis­trac­tion and in­crease the work­load dur­ing an in­struc­tional flight, which may be why nei­ther in­struc­tor at­tempted to con­tact an ATS other than Wy­combe Tower. The col­li­sion was out­side of the pro­mul­gated area for Farn­bor­ough North LARS and was be­low their 1,500ft amsl al­ti­tude re­stric­tion for pro­vi­sion of a Traf­fic Ser­vice. The busy airspace and re­stricted radar cov­er­age in the area was such that any of the pos­si­ble ser­vice providers would prob­a­bly only have been able to of­fer, at best, a Ba­sic Ser­vice.

‘As nei­ther air­craft was elec­tron­i­cally con­spic­u­ous to the other, the only avail­able method of col­li­sion avoid­ance be­tween the two air­craft was “see and avoid”. There are con­sid­er­able and well un­der­stood lim­i­ta­tions to “see and avoid” and there was no ev­i­dence to sug­gest that the oc­cu­pants of ei­ther air­craft had seen each other in time to avoid the col­li­sion. The C152 was de­scend­ing from above the Cabri on a sim­i­lar course and gain­ing ground. The (C152’s) planned ex­er­cise was 8.1 Glide De­scent. How­ever, it was de­scend­ing at a higher av­er­age rate and air­speed than would be ex­pected for a best an­gle glide, so it is pos­si­ble that a higher air­speed was used, with a cor­re­spond­ingly in­creased rate of de­scent. The an­gle at which the air­craft were clos­ing was such that nei­ther was in the field of view of the other un­til per­haps a few mo­ments be­fore the col­li­sion.’

The dam­age sus­tained by the Cessna in­di­cated that ini­tial con­tact had been be­tween its right wing and the Cabri’s main ro­tor blades, with the wing be­ing struck from be­low, and the rel­a­tive ge­om­e­try of the two air­craft and the 110° an­gle of the cuts sug­gest that the C152 was slightly ahead of the he­li­copter im­me­di­ately be­fore the col­li­sion and that one or both air­craft may have been ma­noeu­vring im­me­di­ately prior to the col­li­sion. ‘[Thus] the pos­si­bil­ity of sud­den eva­sive ac­tion can­not be dis­counted,’ says the AAIB.

Its re­port con­tin­ues: ‘Fixed­wing air­craft with for­ward-mounted en­gines have a re­stricted view ahead and be­low the flight­path. As a con­se­quence, to see into the blind spots and to de­ter­mine that the area into which it is de­scend­ing is clear, an air­craft would have to be ma­noeu­vred, gen­er­ally by con­duct­ing a se­ries of shal­low turns. Radar data in­di­cated that, when the Cessna was 1,950ft above the Cabri, it was only 0.5nm be­hind it. A sim­ple as­sess­ment in­di­cates that, if the C152 was straight and level with zero pitch an­gle at 2,000ft above the he­li­copter, it would have to have been at least 1.9nm be­hind for the pi­lot to have had any op­por­tu­nity of de­tect­ing a pos­si­ble con­flict with­out ad­di­tional ma­noeu­vring. In the ab­sence of a turn, the pi­lot would need to have pitched the air­craft at least 24° nose-down to have had any chance of ob­serv­ing the he­li­copter. The sit­u­a­tion whereby ad­di­tional ma­noeu­vring would have been re­quired (by the Cessna) to bring the he­li­copter

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.