Karachi Squall Line: A Legal Approach That Should Not Be Flown
Jinnah International Airport, Karachi, 15 July, 1600 PST. A monsoon squall line is crossing the field. The current METAR: OPKC 151100Z 25028G45KT 1200 TSRA SCT015 FEW030CB OVC080 27/25 Q0996 NOSIG. On radio the tower adds that a departing crew reported a 30 kt loss of headwind at 1 NM on final for runway 25L (magnetic heading 254°), and that lightning is visible in the cloud to the north-east. ATIS gives touchdown RVR for runway 25L as 1,400 m. The inbound aircraft is an A320 with a 33 kt gust-inclusive crosswind limit, CAT I minima (DH 200 ft, RVR 550 m), and company rules prohibiting an approach when a convective cell is within 5 NM of the approach path or when severe wind shear is reported. Fuel allows one approach and 25 minutes of holding before diverting to Nawabshah, 150 NM inland, where the METAR shows 240/12 kt, 10 km, no significant cloud.
1. Quantify the wind components and determine which element of the report actually threatens the approach.
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Runway 25L is 254° magnetic, wind 250°, so the angle is only 4°. Headwind component 28 × cos 4° ≈ 27.9 kt, crosswind 28 × sin 4° ≈ 2 kt; with the gust, crosswind is 45 × sin 4° ≈ 3 kt. Against a 33 kt limit the crosswind is a non-issue - the gust is almost straight down the runway.
The threat is therefore not crosswind and not visibility. RVR 1,400 m is far above the 550 m CAT I minimum, so the approach is legal on paper. What matters is: TSRA with a cumulonimbus overhead, lightning in the vicinity, and a reported 30 kt headwind loss at 1 NM final - that is severe low-level wind shear, almost certainly a downburst/microburst given the 27/25 °C spread with heavy rain. Losing 30 kt of a 28 kt headwind on short final means the aircraft could momentarily face a tailwind component with a large airspeed loss at low height, where there is no recovery margin. Both the company 5 NM convective rule and the severe-shear rule are triggered. Conclusion: the limiting factor is wind shear under an active thunderstorm, not the crosswind or RVR.
2. The crew argues the approach is legal and wants to try. Give the commander's decision and the reasoning to be recorded.
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Legal is not the same as safe. The approach must be rejected because two company prohibitions are simultaneously satisfied: a convective cell over and near the field (well within 5 NM of the approach path) and a reported severe wind shear. The METAR also carries NOSIG, meaning no significant change is expected in the next two hours - the squall is not about to clear.
The RVR of 1,400 m is measured in heavy rain and is likely unrepresentative of the touchdown zone during a downburst. The 27/25 °C temperature/dewpoint spread is small, indicating a saturated, rain-cooled downdraft environment in which further microbursts are likely.
The correct decision is to hold clear of the cell at a safe distance, or divert immediately, and to advise ATC of the reason. The commander should record that the go-around decision was based on reported severe wind shear and thunderstorm proximity, not on minima. A PIREP and a report to the meteorological office on the 30 kt shear should follow, since the shear was reported by a departing crew and must be passed to subsequent arrivals.
3. Evaluate the fuel plan and decide whether to hold at Karachi or divert now.
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The fuel available is one approach plus 25 minutes of holding, after which the aircraft must go to Nawabshah, 150 NM away. At roughly 220 kt groundspeed that diversion is about 40 minutes' flying, plus an approach and landing - so the 25 minutes of holding is not free time. If the crew holds for the full 25 minutes and then diverts, they arrive at Nawabshah with essentially no margin above final reserve, and Nawabshah itself could be affected if the squall line propagates inland.
Since the destination is forecast to remain in thunderstorm conditions for at least two hours (NOSIG), holding is very unlikely to produce a landing opportunity. The rational use of the fuel is to divert early, while the margin is intact, rather than to burn it waiting for a clearance that the METAR says will not come. Holding is justified only if a specific, imminent improvement is expected - for example a reported gap in the line or a nowcast from the meteorological watch office.
Decision: divert to Nawabshah now, or hold for at most one short orbit while confirming the state of the alternate; do not commit to the full 25 minutes.