Dreadful Lady over the Mekong Delta: RAAF Canberras in the Vietnam War by Bob Howe

Author:Bob Howe
Language: eng
Format: epub
Publisher: Big Sky Publishing
Published: 2023-06-15T00:00:00+00:00

Professional lapse – density altitude

Aside from turbulence experienced in rough weather, tropical conditions also posed quite a different and unique challenge for No 2 Squadron in seeking to bomb as precisely as possible. Despite the average land height in the Mekong Delta being not more than 5–10 metres above sea level, a combination of both high temperatures and high humidity could generate, at sea level, a ‘density altitude’ of up to 2,000 feet.279 This feature had major consequences for helicopter and light-aircraft engine power and flight performance, particularly when required to carry heavy loads. For unwary operators, this high density altitude took these aircraft close to, or over, their all-up weight for take-off.280

It was only when a thorough assessment of No 2 Squadron’s bombing accuracy history was carried out by the author in late 1969, that a significant undershoot was discovered across the total of all recorded Magpie bomb drops, up until that time. In conjunction with No 2 Squadron Navigation Leader, Squadron Leader Frank Lonie, an analysis was conducted of the reasons for this error. The conclusion was that the squadron had either been unaware of, or had ignored, the density altitude effect on air pressure measurements. This had a direct impact on the Canberra’s bombing altitude.

If a density altitude correction was not applied to the altimeter reading, the effect was equivalent to raising the actual bombing altitude above the target. Squadron aircrews, when positioning for their final bombing runs, were fastidious in ensuring their altimeters were adjusted to take into account the atmospheric pressure reading given by the FAC. This was normally obtained from the control tower operator by the FAC when departing his home base.281 Known as QNH, this pressure setting was manually adjusted by the pilot, who wound a small knob located on the altimeter dial on the front instrument panel. This brought the pressure setting in millibars, shown in a small window at the bottom of the altimeter dial, into alignment with the given QNH.282

QNH would be set on the Canberra’s altimeter before departure from Phan Rang, so that the pilot would have an accurate indication of the airfield height, if he needed to return promptly to land. On the climb to cruise altitude, both pilot’s and navigator’s altimeters would be cross-checked and adjusted to a global standard pressure setting (1013.2 millibars or 29.92 inches) when passing 10,000 feet and, on the descent into the target area, they would be adjusted back to the local QNH figure. QNH was a crucial input to the altimeter setting before commencing a bombing run, assuming of course the target’s altitude was also known.

Across the flooded southern part of the Mekong Delta, target height was known fairly precisely, due to the flat nature of the terrain. As part of their bombing briefing given by the FAC, which included target height, almost invariably Magpie crews would be given the QNH from the nearest and only significant air base – Bình Thủy. IV Corps FACs were well aware the Canberra, with

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