Leveraged woes

Leverage in engineering is usually beneficial; much force with little effort. In finance, too, it can create great wealth from nothing by borrowing.

It seems leverage can also act detrimentally in engineering. Some mishaps have had worse ‘leveraged’ outcomes than expected from the initial cause.

Last year’s 777 near-disaster at Heathrow becomes ever more mysterious, since the Air Accident Investigation Board has still not published the definitive cause. Perhaps the known restricted fuel supply to the hp pumps, causing cavitation and gassing, which of itself would not have reduced fuel sufficiently seriously, was ‘leveraged’ by the fact the spill-back lines from the metering valves were fed back to the inlet of the pumps (making many a combustion engineer of light distillate fuel, cringe; they would have fitted a two-pipe system to ensure any gases were lead harmlessly back to the fuel tank).

Although fuel supply was diminished somewhat, four increases in engine power were successfully executed seconds before fade-out to maintain the desired 140 knots. Each time, though, portions of high-pressure gassing fuel may have been spilt back, at only atmospheric pressure (increasing gassing volume enormously) to the inlet of the fuel pumps, so choking the pumps with gases and severely reducing kerosene mass flow to the rapidly fading engines.

Since both engines’ fuel systems are identical, this known single pipe gassing risk, faded both engines simultaneously.

PH Field,

St Albans