Heard the news about the Singapore Airlines flight SQ836 that experienced a temporary loss of power to both of its engines last Saturday?

The flight was bound for Shanghai when it lost power in all its engines at 39,000ft near Hong Kong on May 23, 2015, according to aviation sites AeroInside and The Aviation Herald.

Engine power was later restored, and the flight, with 182 passengers and 12 crew, continued its journey uneventfully to its final destination, Shanghai. However, technicians and engineers could find no anomalies with the aircraft.

First, let us take a deep breath, and understand what the “loss of power” means.

It does NOT mean that the both engines went dead, and the cockpit blacked-out, with the oxygen masks tumbling from the roof. It just means that the thrust provided by both engines was significantly reduced, but the engine was still running.

1. Was the airplane going to crash?!

In the unlikely scenario that both engines do completely go out, the aircraft will NOT belly up and tumble to earth. It will simply glide down.

Commercial aircraft have a glide descend rate of about 3,500ft/min, so at 39,000ft, the pilot has at least 10 minutes to remedy the problem, and guide a plane to a safe landing spot. The glide distance ranges between aircraft, but typically from 39,000ft, you have a distance of about 80km. That is the distance from Tuas to Changi Airport, and back to Tuas again!

2. So what happens in the cabin?

Contrary to popular belief, the cabin will not go black and the aircraft will not experience a full system failure. Systems will continue to work, drawing power from a separate independent generator known as an Auxillary Power Unit (APU). The APU will also provide pressurisation to the Cabin.

However, the hypnotising drone of the jet engines will no longer be there. The engines, which produce a significant amount of noise, will go quiet, leaving an eerie silence in the cabin.

3. How in the world did two engines fail at the same time?

This is mere speculation at this point in time.

However, I believe that investigations will show that the cause of the engine loss of power would be due to High Altitude Ice Crystal Icing.

What happens is a convective weather system pushes warm moist air to high altitudes, where the moisture freezes into tiny crystals so small, neither human eye nor weather radars can detect it. They pose no structural threat to the aircraft.

However, they can accumulate on the blades inside the engine, causing the blades to loose efficiency, and thus power. However, when the aircraft descends to a lower altitude where the ambient temperature is warmer, the ice will melt away and the engine would work normally again.

The scenario fits that of SQ836 perfectly, where it experienced bad weather before the engine problems were detected at 39,000ft. It later descended to 26,000ft when full power was restored.

4. How to avoid this?

Ever experienced a flight where the pilot announces a few added minutes to a flight to “avoid bad weather”, before hearing grumbles from fellow passengers in the cabin?

Well now we know why the pilot does this. Though the aircraft is designed to survive bad weather, the safest option is to avoid bad weather systems totally, because we never know what may happen.

Related article:

S’pore to Shanghai SQ836 flight landed safely after experiencing temporary loss of power to both engines mid-air

Editor’s note: Alfred Yeo has a postgraduate degree in Aerospace Engineering.

Top photo from Wikipedia. 

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