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Use of Selected Altitude by ATC

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Category: Level Bust Level Bust
Content source: SKYbrary About SKYbrary


The provision of the selected altitude set by a crew to the controller gives him/her the ability to intervene where, for whatever reason, the selected altitude does not match the clearance. This greatly reduces the chance of a level bust.

Benefits of Mode S

Elementary Mode S allows selective interrogation of aircraft providing the potential to eliminate, amongst other things, the synchronous garbling of replies. Additionally, elementary Mode S provides for down-link of the aircraft identification.

Enhanced Mode S provides the functions of elementary Mode S, and also enables additional down-link of airborne parameters (DAPs), including ground speed, indicated airspeed, heading and the selected altitude entered by the crew into the Mode Control Panel (MCP) or Flight Control Unit (FCU), that feeds into the aircraft’s autopilot system.

Mode Control Panel
Figure 1. Typical Mode Control Panel showing selected altitude of 23000 ft

Down-link of Airborne Parameters

Improved integrity of radar surveillance and the availability of Mode S DAPs have enabled the development of controller support tools that have provided benefits to the safety and efficiency of ATC operations. The DAPs of suitably equipped aircraft are displayed to the controllers in the track label and/or in flight lists, such as vertical stack lists. DAPs, such as ground speed, indicated air speed, and magnetic heading can also be displayed in the track label (usually in line 3) either individually, or in combination.

Colour is used in some ATC systems to distinguish the selected altitude (level) from actual flight level (Mode C) and from other flight levels displayed in the track label. In the example provided by Figure 2 below the selected altitude (level) is displayed in orange/yellow colour.

However, in certain specific circumstances, the generation of label overlap may create adverse impact, and in such situations there may be a need for selected DAPs to be inhibited.

DAP display airtraffic control
Figure 2. Example of DAP display at controller working position, NATS

Selected Altitude - Benefits & Limitations

Selected altitude data is presented as either a flight level or an altitude, depending on surveillance system settings. For ATC and air ground communication purposes, the generic phrase ‘Selected level’ is often used to encompass data presented as either an altitude or a flight level.

The selected altitude DAP has the potential to help in level bust prevention as it provides the opportunity to alert ATC if there has been any misinterpretation of the altitude/level clearance. The analysis of level bust occurrences carried out by NATS showed that the following factors are being preventable:

  • Correct pilot read-back followed by incorrect action;
  • Incorrect pilot read-back by correct aircraft;
  • Pilot read-back by incorrect aircraft.

A further safety study, carried out recently by NATS, found that, following the implementation of Modes S and DAP, overall there had been a 63% reduction in the level of risk exposure in UK airspace associated with the above causal factors, expressed as the severity of the consequent level bust.

Air traffic controllers need to be aware of the limitations of selected altitude data down-linked by Mode S. The data presented shows intent only, and therefore cannot be used for separation purposes. Additionally, there may be occasions where despite the flight crew complying with the ATC clearance, the displayed selected altitude is at variance, for example:

  • Along SID/STARs with vertical restrictions where pilots may select the final cleared level, and utilise the aircraft flight management system to achieve the vertical constraints.
  • On final approach, at which point pilots may pre-select the missed approach point (MAP) altitude. The ATC system should normally inhibit the display of the selected altitude in the track label before this occurs to avoid any confusion. If the system detects that the aircraft has broken off the approach and is climbing then the selected altitude will automatically be displayed again providing the controller with the aircraft’s revised intention.
  • When the aircraft is being flown manually, in particular in response to vertical avoiding action manoeuvres.
  • Where there is an incorrect barometric pressure setting in the FCU.

Whilst it is considered that the display of selected altitude is an obvious safety enhancement, in addition to the limitations listed above, human workload limitation and time delays incurred for input of the information into the MCP/FCU must be taken into account. Therefore, the requirement for aircrew to read-back all clearances and for controllers to check the read-back shall not be affected by the display of selected altitude.

In order that the safety benefits of using selected altitude are exploited without adverse impact, there is a need for standardised ATC procedures and phraseology to be promulgated. Additionally, aircrew need to be aware that a controller may challenge the selected altitude reading if there is any doubt over the cleared level. The phraseology used should not state the ‘incorrect’ level as observed on the situation display, as this could reinforce the error and further confuse the pilot. In compliance with this safety consideration the following phrase is employed in UK: “(Callsign), CHECK SELECTED LEVEL. CLEARED LEVEL IS (correct cleared level)”.

Selected altitude data is generated directly from the pilot settings and manipulation of the aircraft MCP or FCU; consequently, there is not known to be any independent means of deselecting potentially corrupt data. A review of UK Mandatory Occurrence Reporting data from the period 2005 - 2009 has not found any instances of corruption between aircraft MCP/FCU setting and the DAP displayed to the controller. However, the review identified 35 instances of autopilot failure to capture the selected altitude entered into the MCP/FCU. Therefore, regardless of the display of selected altitudes, controllers need to remain alert to the potential for autopilot failure to capture set levels and subsequent level bust.

The display of selected altitude to controllers can have additional safety benefits, for example in highlighting call-sign confusion. Also, further enhancements can be made because at the moment prevention requires the controller to observe/monitor the selected altitude and compare it to the cleared level. There is no guarantee that a controller can carry out such a task at all times and incorrect settings may still occur. With the progressive introduction of electronic flight data the ATC systems could generate in the near future automatic alerts to the controllers to a discrepancy, thus reducing risk even further.

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