Safe operating capacity at smaller aerodromes

An annual total is demand, not capacity

Two aerodromes can record the same number of movements a year and carry a different risk picture. One spreads its traffic evenly across the season. The other concentrates training sorties into a handful of good-weather weekends, so the number of aircraft sharing the circuit at the busiest hour is far higher than the annual average suggests.

That difference is invisible in an annual count. A movement number becomes useful for a safety decision only once it is translated into the period in which pilots actually experience other traffic: a busy day, a busy hour, and the minutes an aircraft spends in the circuit or on the runway during it.

Four methods, integrated rather than averaged

A defensible answer to a capacity question draws on four complementary methods. Circuit occupancy converts the annual total into a peak-hour rate and estimates the probability that several aircraft are in the circuit at once. Occurrence history tests whether reported events and near misses are consistent with that estimate. Runway occupancy time checks the physical throughput of the runway itself, independent of any probability model. Human factors work tests scenarios that a probability model cannot capture, in particular an unexpected or non-radio aircraft joining a circuit that already has known traffic in it.

None of the four methods stands alone, and none of them is averaged with the others to reach a single score. They are integrated through release gates: a strong result on runway throughput cannot cancel an unresolved finding from the human factors work, and a quiet occurrence history does not by itself clear a circuit occupancy estimate that is trending toward its upper band.

From an annual total to a peak-hour picture

The conversion from an annual movement total to a peak-hour occupancy estimate runs through a short chain of local factors: the share of annual movements that falls in the high season, the number of days in that season, a busy-day factor, a peak-hour factor, and the number of operating hours in the day. Multiplying the resulting peak-hour rate by how long an aircraft typically occupies the circuit gives an expected occupancy figure, which is the input to a standard probability calculation for the chance that three or more aircraft share the circuit at once.

Every term in that chain is a local input, sourced and dated, not a constant borrowed from another aerodrome. The worked example in the whitepaper below shows the full chain applied to a real case, with every intermediate result shown so another analyst can rerun it against their own numbers rather than take the conclusion on trust.

What this method does not do

This is not an approval process, and it does not replace a formal safety assessment where one is required. It also does not replace NorthSky’s own Safe Movements screening tool, which asks whether the arrangements supporting an existing traffic level look proportionate and takes about ten minutes. The screening tool deliberately does not calculate or approve a movement number.

The four-method approach in the whitepaper is what a full assessment looks like once screening has raised a question the ten-minute tool cannot answer: it needs local traffic and peak data, measured circuit residence time, runway occupancy evidence, an occurrence history, and a documented, accountable risk decision, not a single input field.

Moving from a screening result to a decision

An aerodrome operator, safety manager or club committee that has run the screening tool and wants to know what a fuller answer would take can use the whitepaper and its companion methodology as a starting point: they set out the calculations, the validation tests and the decision gates in enough detail that a competent analyst can apply the same method to their own data.

Where the traffic mix, the runway configuration or a proposed change to the operating concept raises a question the published method cannot settle from a reader’s own desk, the next step is a site-specific assessment rather than a heavier reading of the same document.