The human mind is fallible and error can occur for many reasons, for example, from a misheard message, from memory slip, or from incorrect appreciation of the situation.
Error is particularly likely in certain circumstances, especially when there is pressure to complete a task quickly (e.g. to expedite departure or during an emergency or abnormal situation), but may also occur in normal everyday situations.
Error in aviation can have severe consequences and the cross-checking process is used wherever possible to eliminate error.
Cross-checking and the Pilot
The cross-checking process is a vital element of a pilot's duties, particularly in a multi crew situation where the roles of the two pilots are defined as Pilot Flying (PF) and Pilot Not Flying (PNF). The PNF - alternatively referred to as the Pilot Monitoring - has responsibility for monitoring the actions and awareness of aircraft control of the PF.
Whilst the monitoring role of a PNF must not be limited to specific parts of flight crew duties, Company SOPs should include a minimum list of defined actions which are to be cross-checked, for example:
Cross-checking is equally important for the ATCO, and comprises two elements:
Cross-checking the actions of pilots
Where possible, the controller should monitor the actions of the pilot, either by reference to the situation display or by visual observation, to ensure that instructions are followed correctly.
The extent to which a controller can cross-check the actions of pilots depends on his/her workload; however, every effort should be made to do so in situations where error is likely to occur. For example, when the pilots are dealing with an aircraft unserviceability, or when the pilot appears to be inexperienced, confused, or have limited language ability. A particular example of a situation where monitoring by radar or directly may be conducive to safety is the execution of issued VFR clearances in airspace such as Class 'D'; in this situation, loss of separation against IFR traffic can occur due to poor situational awareness of the IFR aircraft flight crew, who might wrongly assume that they benefit from ATC-controlled separation from VFR traffic as well as from other IFR traffic.
Controllers should pay particular attention to aircraft manoeuvring on the ground near runway hotspots and to potential conflicts which can arise in the air when intersecting runways are in use simultaneously and this involves intersecting approach, missed approach or take off flight paths.
System support can be used to help controllers with performing this task. Examples of this are various monitoring tools, e.g. for a potential or actual level bust, horizontal deviation, the downlink of Mode S selected level, etc. Nevertheless, controllers should be aware that such tools are not supposed to replace the existing ATC procedures.
Cross-checking the actions of colleagues
Cross-checking is a normal part of the duties of an ATC Assistant if these exist; otherwise, controllers rarely have the free capacity to monitor the duties of other controllers and such action could not be expected to form part of their duties. Nevertheless, the following areas are important:
When there are two controllers assigned to a sector, the communication with aircraft is normally done by the executive controller. The planner controller however also monitors the radio exchanges (to the extent possible) so that they can detect lapses, incorrect readbacks, etc.
Also not official and subject to personal workload, a tower and an approach controller (or a tower and a ground controller) may monitor the other controller's frequency e.g. to make sure an agreed coordination is appropriately communicated to the aircraft.
Controllers taking over responsibility for a sector have much information to absorb and the potential for error or oversight is high. The controller going off duty should monitor the actions of their replacement for a few minutes after hand-over to ensure that neither has overlooked any significant aspect of the prevailing traffic situation and to be available to deal with any questions that might arise;
Inexperienced controllers or controllers who are new to their positions may not become fully proficient for some time. Appropriate mentoring procedures should be in place until their unaided performance is assessed as satisfactory.
When a controller is dealing with an abnormal situation, e.g. an aircraft emergency or very high density traffic, the enlistment of any off-duty controllers to assist can be an important safety net.
Accidents & Incidents
Events in the SKYbrary database which include Ineffective Monitoring as a contributory factor:
On 21 March 2022, an Airbus A320 level at FL 360 lost separation with another Airbus A320 which continued its descent beyond its cleared level. A predictive conflict alert prompted the controller to issue multiple calls confirming the clearance limit but with no response so when both aircraft tracks crossed at FL 360, lateral separation was reduced to 3.8 nm. It was concluded that the immediate cause of the conflict was the failure of the descending aircraft to respond to ATC alerting calls but that its origin was an undetected incorrect readback of the descent clearance.
On 24 October 2021, a Bombardier DHC8-400 inbound to Belagavi initially advised to expect a non-precision procedural approach to runway 08 was subsequently instructed and acknowledged clearance for an equivalent procedural approach to runway 26. An approach to runway 08 was then flown without ATC intervention or pilot error recognition but with no actual consequences. The error was attributed to pilot expectation bias and distraction and controller failure to order a go-around after eventually realising what was happening. The context which had facilitated the errors was considered to be procedure and performance inadequacy at both the aircraft operator and ATC.
On 8 June 2022, a Boeing 757-200 making a night visual approach to Tulsa inadvertently landed on runway 18R instead of 18L as pre-briefed and cleared. ATC did not intervene and neither pilot realised the error until the Captain realised that having intentionally landed long because the turn off was at the end of the much longer 18R there was less runway ahead than he had expected. Although both pilots reported not being fatigued, it was concluded that lack of recognition of their error suggested otherwise and probably facilitated plan continuation bias aided by inability to efficiently integrate available information.
On 6 November 2018, an Airbus A340-600 in the cruise northbound over the Swiss Alps received an overspeed warning after encountering an unexpected wind velocity change but the crew failed to follow the prescribed response procedure. This led initially to a climb above their cleared level and further inappropriate actions were then followed by PAN and MAYDAY declarations as control of the aircraft was briefly lost in a high speed descent to below their cleared level. The operator subsequently enhanced pilot training realism by providing it in a simulator configured for the aircraft variant operated and introduced ‘upset recovery training’.
On 1 December 2020, a Viking DHC6-300 crew departing Wobagen set asymmetric power in response to directional control difficulties but this did not prevent the aircraft subsequently veering off the runway and into a ditch. Both engines were found to have been operating normally and with failure to complete takeoff checks resulting in the initial setting of asymmetric power. This was then followed by an unsuccessful attempt to regain directional control on the wet and deteriorated clay/silt runway surface without reducing power. Both pilots were experienced in the use of small airstrips generally and with Wobagen in particular.