What happened
During a flight operating within reduced vertical separation minimum (RVSM) airspace, the flight crew identified a discrepancy between the captain's and the first officer's altimeters. Upon reaching the assigned cruising altitude of FL330, the first officer's altimeter indicated a height approximately 340 ft higher than the captain's instrument. This altitude difference was observed to persist throughout the cruise phase, with the first officer's reading remaining consistently higher by roughly 300 to 390 ft. While the crew noted minor variations in airspeed and Mach numbers, these remained within the manufacturer's permitted error limits.
The investigation
The investigation focused on the maintenance performed two days prior to the flight. It was discovered that during a scheduled engine wash, maintenance personnel did not utilize the required Airwork 737-300/400 ground run checklist, instead relying on memory. Consequently, the pitot-static probe covers were not removed, and the pitot-heat circuit breakers were not opened before engine start.
Because the operator was using locally sourced polyvinyl chloride covers rather than the manufacturer-recommended Kevlar™ material, the covers melted due to the heat from the pitot-heating system. This left residue on all four pitot-static probes. Although the probes were cleaned following the melting event, the ATSB found that residue remained on the surfaces. While the flight data did not directly link this residue to the specific altitude error during this flight, the presence of the residue posed a risk to instrument accuracy.
Findings
- The primary cause of the altitude discrepancy was the failure to remove pitot-static probe covers during maintenance.
- The use of non-compliant polyvinyl chloride covers increased the likelihood of the covers melting onto the probes when exposed to heat.
- Maintenance personnel performed the engine wash task in stages with insufficient communication, which contributed to the oversight.
- The flight crew did not notify Air Traffic Control (ATC) of the altitude discrepancy, despite the error exceeding the 200 ft limit allowed for RVSM operations, because they believed the first officer's altimeter was the accurate instrument.