Overview
An investigation into a holding event revealed that the flight crew selected an incorrect hold speed during descent planning. The VNAV profile in the flight management computer (FMC) initially calculated holding between FL 150 and FL 160. When selecting a target speed, the crew verified the FMC-provided speed by comparing it to the flaps-up manoeuvre speed. However, the flight crew training manual stated that the flaps-up manoeuvring speed guaranteed full manoeuvre capability to stall warning activation at low altitudes, and the crew were unaware that a different speed calculation verification was required for altitudes above FL 200.
Speed Calculation and Selection
Had the crew recalculated the hold speed for FL 220 using the FMC, it would have provided a target hold speed of 240 kt. The flight crew likely lacked adequate understanding of how the FMC calculated the target hold speed and did not understand the use of the "best speed" option on the FMC hold page. Using best speed would have provided a hold speed appropriate for the actual aircraft weight, altitude, and configuration. The selection of an incorrect hold speed resulted in the aircraft entering the hold with a selected speed 15 kt below the required speed, reducing the margin against stall warning activation.
Procedural Gaps
Neither the operator nor the aircraft manufacturer provided procedures or guidance stating that hold speed should be re-evaluated when the hold level changes from the planning stage to execution. Consequently, the flight crew lacked the requisite knowledge to identify the need to re-evaluate the selected speed. There was no procedural prompt to re-evaluate the speed for the higher level, so the need relied entirely on the crew's knowledge of the higher speed requirement above FL 200.
Crew Recognition of Low Speed
As the aircraft entered the holding pattern, a delay in the autothrottle system detecting and effecting speed changes allowed the speed to reduce to 220 kt—below both the selected speed of 225 kt and the minimum manoeuvring speed of 223 kt. These speeds, along with the stall warning activation speed, were displayed on the primary flight display (PFD), but the flight crew did not detect the speed reduction as their attention was on other operational tasks. Not noticing a visual indication within one's scan can occur when attention is focused elsewhere, a phenomenon known as change blindness.
Findings
The investigation highlighted that decision errors, such as not re-evaluating the hold speed, can result from a failure to detect a problem when no cues are present. A requirement to re-evaluate the speed for a higher hold level would likely have prompted the crew to reselect the correct speed, providing an adequate margin above the minimum manoeuvring speed.