A FlySafair jet had to make an emergency descent from 10km when damage to an air duct made it impossible to pressurise the cabin, according to a preliminary serious incident report from the Civil Aviation Authority.
The crew of the Boeing 737-400 broadcast a mayday after the cabin pressure alarm sounded during a flight from Cape Town to East London on Sunday, November 22 2020, the report said.
Thirty-four minutes later, after a landing in George that was hampered by a broken radar transponder on the plane, one of the 158 passengers needed medical treatment, three had nosebleeds and five had severe ear pain.
The CAA accident investigation department said it was looking into “other aspects” of the incident “which may or may not have safety implications”.
It said it took cabin crew three attempts before two doors were properly closed before take-off at 7.46am, and FlySafair failed to preserve evidence on the cockpit voice recorder before the plane was flown to OR Tambo airport in Johannesburg.
After take-off, “the crew had no lateral navigation and vertical navigation”.
“The throttle hold stayed on after take-off, and the autothrottle was disconnected manually. For the remainder of the flight, there was no autothrottle available.”
According to Wikipedia, “the autothrottle allows a pilot to control the power setting of an aircraft’s engines by specifying a desired flight characteristic, rather than manually controlling the fuel flow”.
The traffic collision avoidance system on the 29-year-old FlySafair aircraft, which reduces the risk of midair collisions, was inoperative during the flight.
And “during the initial stages of the [emergency descent to about 3km], the thrust levers retarded to low idle”, setting off cockpit alarms including one that warned the pilots the aircraft’s speed was too low.
The website b737.org.uk, which contains contributions from pilots and engineers worldwide, says: “The minimum engine speed for all flight phases is high idle, which varies with flight conditions.”
The website says the average high-idle setting is 32% of full power, and low idle is normally used only on the ground. During the FlySafair’s descent, low idle produced power at about 30%.
In its safety recommendations, the preliminary report said the FlySafair fleet should be inspected to check the condition of the air duct that failed on the Cape Town-East London flight.
It said FlySafair should warn flight deck crew about the consequences of radar transponder failure, which meant the aircraft’s altitude and speed data were not available to air traffic controllers in George.
The airport in the southern Cape town did not have primary surveillance radar and relied on a secondary system which uses planes’ transponders. But the 737-400 had experienced “transponder-related activation malfunction”, said the preliminary report.
The drama in the cockpit began shortly after take-off, when an air traffic controller asked the captain and first officer to restart the transponder because it was not transmitting altitude information. They did so, without success.
When they reached cruising altitude, the pressurisation alarm sounded because cabin altitude had reached 3km when it is supposed to stabilise at 2.4km. It was rising at 213m per minute.
At sea level, oxygen makes up 20.9% of air, while at 2.4km it exists at only three-quarters of that level and at 3km the oxygen level is down to 14.3%. On the summit of the 5.89km Mount Kilimanjaro, Africa’s highest mountain, it is 10%.
After deciding to land at George rather than return to Cape Town, “attempts to reduce the cabin [altitude] rate were unsuccessful and an emergency descent was executed”, said the incident report.
“Aerodrome rescue and firefighting personnel moved into position after being alerted by air traffic control. The aircraft touched down at 8.40am.”
The plane involved in the midair emergency, with registration ZS-OAF, was manufactured in 1991 and had flown for 65,823 hours (equivalent to 7½ years in the air) at the time, said the CAA.
It had undergone its last safety inspection on October 27, and its airworthiness certificate was due to expire on April 30 2021.
Neither the 37-year-old captain nor the 31-year-old first officer had been involved in any previous incidents.
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