Eighty-five Metres of Margin on a Twenty-five-Minute-Old Report
Flight 61, a twin-jet (MLW 64 500 kg), is planned to land at Northfield (KNFL) at 61 500 kg on runway 27: LDA 2 300 m, touchdown-zone elevation 620 ft. The AFM at 61 500 kg, flaps 40, sea level, ISA, still air gives 1 600 m dry; the contaminated-runway table gives 2 090 m at sea level for RWYCC 3. The operator applies a 6 % increment for elevation and takes no wind credit. The runway condition report, issued 25 minutes ago after 40 mm of rain, gives RWYCC 3 with 6 mm of water over 40 % of the length, following a preceding regional jet that reported 'medium'. Wind 260/18G30. The only other runway, 18/36 (1 900 m), has no current report. The PAPI on runway 27 is NOTAMed unserviceable. The destination alternate, 25 minutes away, is dry with a 2 700 m runway.
1. Compute the required landing distance and decide whether the flight can be dispatched to land at KNFL. Which number is the weakest in your calculation?
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Calculation:
- AFM contaminated data at 61 500 kg, sea level: 2 090 m
- Elevation increment: 2 090 × 1.06 = 2 215 m
- No wind credit is applied, so this stands against LDA 2 300 m → 85 m of margin (3.7 %).
So it is dispatchable - but only in the narrowest legal sense, and every number in the chain is soft:
| Item | Why it is weak |
|---|---|
| RWYCC 3, 25 min old | It is an observed condition in continuing rain. Water depth can only reasonably be expected to increase; the report does not describe the touchdown zone where braking is most critical. |
| '6 mm over 40 %' | An average over a percentage of the length. The zone where the aircraft touches down may be deeper. |
| ISA, sea-level table | Warm-day corrections would increase the distance; nothing in the scenario shows the temperature was checked. |
| Gust spread 12 kt | Lengthens the float and increases the touchdown speed dispersion; the headwind credit is deliberately not taken, which is the right call. |
The weakest single number is the age and volume of the water in the RCR - the margin is smaller than the uncertainty in the input.
Correct framing: the dispatch check does not absolve the crew. The pre-landing assessment by the crew, using the current report and their own braking, is the operative check, and the dispatcher must say so explicitly rather than implying that 85 m of paper margin is a clearance to land.
2. Fifteen miles out, the tower reports a new inspection: RWYCC 2, 12 mm of water over 60 %. What are the options and what do you recommend?
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Recalculate first: the AFM table gives 2 540 m at sea level for RWYCC 2 → 2 540 × 1.06 = 2 692 m against an LDA of 2 300 m. Landing on runway 27 is now not possible.
Options:
- Divert to the alternate - dry, 2 700 m, 25 minutes away. Uses the fuel already carried for it. This is the recommendation.
- Hold for drainage and a new report - 20 minutes of holding at roughly 2 300 kg/h is about 750 kg of fuel, with no guarantee: in continuing rain the report is more likely to degrade than improve, and the second approach would be on the same water.
- Runway 18/36 - 1 900 m with no current condition report. An unquantified contaminated-runway distance cannot be computed, so it cannot be used.
- Land long / use full LDA - not a legitimate mitigation. The certified contaminated data is what it is.
What to say to the crew: give them both sets of numbers (2 215 m for RWYCC 3, 2 692 m for RWYCC 2) so 'we're almost there' is not being weighed against a figure they have not seen. The decision to divert is theirs in the air; the dispatcher's job is to make the diversion the easy option by confirming the alternate's weather, runway and handling now rather than after the call.
3. The crew asks whether they can 'have a look and go around if it isn't comfortable'. Assess that plan against the runway state and the fuel.
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It is not free. Three separate costs:
- Fuel. From 15 nm out the aircraft must carry: one approach, a possible go-around (about 250–400 kg), a second approach or a diversion of 25 minutes (roughly 1 000 kg), plus the final reserve. If the fuel state does not cover a go-around plus the diversion plus the reserve, the 'look' is not an available option and the diversion must be made now, at altitude, with fuel in hand.
- The runway after the go-around. The aircraft has just flown through the same rain; the water is still there and possibly deeper. A go-around buys a second look at a runway that has not improved.
- The go-around itself on a flooded runway. A late go-around or a rejected landing from low speed means the aircraft is relying on reversers and spoilers on a surface with degraded friction, and directional control with a 12 kt gust spread is the risk, not just the stopping distance.
Recommendation: divert now, or accept a hold only with a defined time limit and a defined fuel floor at which the diversion is flown regardless. Never accept an open-ended 'have a look'. Note also that the PAPI is out of service, removing the only glideslope cue on a runway with no vertical guidance issue - a small thing that becomes less small when the surface is flooded.