“… use of ‘factored’ data for landing at takeoff alternate …”
EU-OPS 1.400 requires that ‘Before commencing an approach to land, the commander must satisfy himself/herself that, according to the information available to him/her, the weather at the aerodrome and the condition of the runway intended to be used should not prevent a safe approach, landing or missed approach, having regard to the performance information contained in the Operations Manual.'
Many interpretations of this use the dispatch factors for the destination; the logic being that this is an equivalent level of safety.
With FOLD, the data may result in similar landing distance to the dispatch factored distances; however, if FOLD assumes reverse (check the small print) the slightly higher risk of a reverse failure in comparison to the dispatch case could be balanced by the infrequent use of FOLD for a landing at destination. FOLD is usually based on the minimum factor (15%) which can (should) be increased.
In principle the above applies for landing at the take-off alternate.
Assuming that this will be an abnormal operation then the assessment of ‘a safe approach and landing’ can be revised by the Commander – balanced by the necessities of the situation, e.g. use the minimum factor for urgent landing.
For increasing levels of emergency then ‘safe’ should be (continuously) reassessed; for an engine failure where FOLD could be invalided, then reference to the abnormal landing distances – landing with failures, should provide guidance for the landing distance required – with or without factors (check the small print).
As the risks increase the landing decision requires a well-balanced judgement (and thus justifiable after the fact); e.g. choosing the absolute minimum distance would consider an unfamiliar airport, landing at max weight, higher speed, etc, not just the failure, all of which could affect the accuracy of the assumed touchdown point in the data.
N.B. when last seen the Boeing ‘actual’ advisory data – pre OLD, has more limiting assumptions, thus less safety margin in the landing distance calculations (really read the small print). An interpretation of this ‘actual’ data is that after applying the minimum factor (15%), the resultant landing distance is only then something which can be achieved by an average pilot. However, when considering the safety aspects of an abnormal takeoff alternate landing, further factors will be required, particularly re the lack of reverse, etc.
Which manufacturers now publish FOLD?