What happened
On March 7, 2017, a Boeing 737-530, registration N-240AT, operated by LC BUSRE SAC, was performing a scheduled flight from Cusco (SPZO) to Lima (SPJC). While cruising at 36,000 feet (FL360) along airway UM548, the crew identified conditions conducive to ice formation and proceeded to activate the engine anti-ice and wing anti-able systems.
Shortly after the activation of the wing anti-ice switch, the aircraft experienced a "BLEED TRIP OFF" on both the left and right sides. This caused the air conditioning packs to disconnect, resulting in a sudden and rapid loss of cabin pressure. In response to the rising cabin altitude, the crew implemented emergency memory items, which included activating the passenger oxygen masks and the passenger oxygen switch to ensure the safety of all occupants.
The crew declared a "MAY DAY" and initiated an emergency descent to 10,000 feet (FL100). During the descent, specifically around 21,000 feet, the air conditioning packs reconnected automatically, and cabin pressurization was restored. The aircraft subsequently continued to Lima, landing safely without any injuries or damage to the aircraft, aside from the deployment of the oxygen masks.
The investigation
The CIAA investigation focused on the sequence of events following the activation of the anti-ice systems. The investigation examined the aircraft's bleed air control and pressurization systems, both of which were found to be fully operational and showed no mechanical failures during post-incident testing.
The investigation also reviewed the Boeing Quick Reference Handbook (QRH), which specifically warns that using the wing anti-ice system above 35,000 feet can trigger a bleed trip off and subsequent loss of cabin pressure. The investigators analyzed the crew's decision-making process regarding the prioritization of the depressurization emergency over the bleed trip procedure.