The Cleared Level Adherence Monitoring (CLAM) warning provides indication to the controller when the transponder-derived level information differs from the controller-input cleared level. The alert is activated automatically when the necessary conditions are met.
The CLAM warning is one of the features that warn the controller about a level bust. This is important for two reasons:
- Only a small deviation (e.g. 300-400 feet) is often enough to cause a loss of separation with another aircraft.
- A prompt reaction will likely quickly restore the prescribed vertical separation minimum and avoid the separation breach altogether.
While some ATS systems consider CLAM to be a safety net, others regard it as an advisory (monitoring) tool as the underlying event (level bust) does not necessarily require an immediate response (as is the case with STCA, MSAW and APW).
The indication on the situation display] may take various forms, e.g. the "CLAM", "BUST" or another string on top of the aircraft label, specific colour for the cleared and/or actual level, etc. The choice of colour has to be conspicuous, e.g. yellow, orange, magenta, red, etc. so that controller attention is immediately attracted.
Examples of CLAM representations: the orange string CLAM above the label (left) and pink background of the cleared flight level field in the label (right). The examples are not taken from a real life system and are for illustrative purposes only.
The CLAM logic normally includes features that reduce nuisance alarms. For example, a warning is normally not displayed if:
- the aircraft is climbing/descending towards the cleared level
- the aircraft`s vertical speed is low (and therefore, there may be consecutive transponder replies with the same level information). If however, the aircraft stops its climb or descend at an intermediate level for a (defined) period of time, the warning will be activated.
- the difference between the actual and the cleared level is below a specified threshold (normally 200 ft).
Limitations of the CLAM warning:
- The reliability depends heavily on the controller's discipline, i.e. prompt and correct input of any change to an aircraft's cleared level.
- It is not available for tracks without Mode C data, e.g. when the transponder is missing/inoperative/off or set to Mode A only.
- It is not available for non-correlated tracks where the controller cannot input the cleared level.
- The activation normally means that a level bust has already occurred. This can be mitigated by:
- Strict application of the read-back and hear-back procedure, which would reduce the risk of e.g. climb or descent to a level that has not been cleared.
- Display of Mode S selected level to the controller. This would warn the controller in good time in case the pilot mis-sets the cleared level.
- Designing CLAM logic to include situations when the aircraft is approaching the cleared level at a high vertical speed. This solution may increase the number of nuisance alarms.