Twenty Days Without Covers — The Blocked Pitot of Puerto Plata

A Boeing 757 chartered from Birgenair sat at Puerto Plata for what investigators believed was twenty days without flying. During that period an engine inspection required a ground run — and investigators believe the engine and pitot covers were not installed before or after the test. The aircraft was then returned to service without the pitot-static verification recommended by the manufacturer's maintenance procedures for a lengthy time on the ground. On the night takeoff, the captain's airspeed indicator was dead at the first officer's "80 knots" call; the captain elected to continue. In the climb, the blocked left pitot line made his indicator read ever-higher airspeed, triggering an overspeed warning while the aircraft was actually approaching a stall — two alarms telling opposite lies. The crew's confusion ended with the aircraft stalled and rolling into the Atlantic about five minutes after brake release. The wreckage — and the pitot tube — were never recovered; the report states the exact reason for the obstruction was never determined, and that the probable source was mud and/or debris from a small insect introduced into the pitot tube while the aircraft was on the ground.

The report's honesty is part of the lesson: the investigators concluded the obstructed pitot tube was not the probable cause — the crew's failure to recognize and recover was — but named maintenance practices as a contributing factor in exact terms: not installing the pitot system covers while the aircraft was on the ground, and the failure to perform return-to-service tests of the pitot/static system after a lengthy time on the ground (JIAA final report, §2.3). Had the recommended verification been done, the report says, the blockage may have been discovered and corrected before flight. A pitot cover costs almost nothing; its absence, uncorrected by the return-to-service check, put a sensor failure in front of a crew at night with 189 aboard. The popular "mud-dauber wasp nest" detail is not in the report's wording — the report says only "a small insect", and marks the cause of blockage as undetermined.

1. Install pitot and probe covers whenever the aircraft is parked beyond the manufacturer's short-stay limit — storage procedures start on day one, not when someone remembers.
2. Ground runs do not suspend storage protection: covers come off for the run and go back on afterwards, as logged steps.
3. Return-to-service after prolonged parking requires the manufacturer's pitot-static system verification — it exists precisely to catch what settled in the tubes.
4. Record cover installation and removal in the aircraft's paperwork so a missing cover is a visible discrepancy, not a silent nothing.
5. Brief crews that any airspeed anomaly at the low-speed calls after ground storage is a reject-and-inspect item — the pitot-static system is guilty until proven clean.

Frequency is not quantified in public sources — but 1996 alone saw two fatal Boeing 757 pitot-static accidents eight months apart, Birgenair in February and Aeroperú in October, the second occurring before bulletins prompted by the first had reached operators (JIAA final report; DGAT Peru AIB report, conclusions).

Erroneous and diverging airspeed indications from the captain's blocked pitot line drove autopilot and autothrottle responses and an overspeed warning while actual speed decayed; the aircraft stalled and struck the Atlantic about 14 NM offshore roughly five minutes after takeoff. All 189 aboard — 13 crew, 176 passengers — died; the aircraft was destroyed on impact, and the wreckage including the pitot tubes was never recovered (JIAA final report, §1.1–1.2).