Shamal Dust, a Squall Line at Destination and GNSS Interference
July, 14:20 local at OKBK. An A320 is planned OKBK–OERK with ETD 14:40 and ETA 15:45. Weather: OKBK METAR 33014G28KT 3000 BLDU 46/09 Q0998, with TEMPO 1500 BLDU; OERK TAF 33018G35KT 5000 TSRA BKN030CB TEMPO 1500 +TSRA SQ. En-route reports give moderate turbulence and dust layers up to FL150. The destination alternate is OEDF, 25 minutes away, forecast 6000 HZ. The aircraft and crew are CAT I only; company CAT I ILS minima to Riyadh runway 33R is RVR 550 m. A NOTAM warns of GNSS interference within 60 nm of the route, and the arrival procedure is RNAV. Crews ahead report convective gust fronts crossing the approach path at OERK and one go-around due to windshear.
1. Interpret the OKBK weather and decide whether the take-off is acceptable.
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Decode it. 33014G28KT: wind 330° at 14 kt gusting 28 kt, a gust spread of 14 kt. 3000 BLDU: 3,000 m visibility in blowing dust. 46/09: temperature 46 °C, dewpoint 9 °C. Q0998: QNH 998 hPa.
Take-off. Runway 33L with wind 330/14G28 gives an almost aligned wind, roughly 14 kt headwind with a 26 kt peak; the crosswind component is only a few knots, so crosswind limits are irrelevant. Visibility of 3,000 m is far above any normal take-off requirement (typically 400 m for a runway with centreline lights and markings, or 125 m if approved for low-visibility take-off), so visibility is not limiting on the ground.
What does matter. (1) A 14 kt gust spread meets the usual low-level windshear advisory criterion of 10 kt or more - brief a windshear escape manoeuvre and expect mechanical turbulence and dust devils near the threshold. (2) 46 °C puts the density altitude near 4,700 ft, so the take-off is performance-limited rather than visibility-limited; confirm the performance figures. (3) Blowing dust with 26 kt gusts means sudden local visibility drops and sand ingestion risk - set clear abort criteria before rolling. (4) The TEMPO 1500 BLDU requires a return/alternate contingency.
Verdict: the take-off is legal and sensible, but only with a windshear brief, an abort brief and a performance check - not as a routine departure.
2. Approaching OERK the crew receives RVR 700 m for runway 33R, wind 330/25G45, CB overhead and a windshear report. How should the decision be made?
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This is a commitment decision, and fuel arithmetic governs it as much as weather.
Fuel first. At roughly 2,200–2,400 kg/h, 30 minutes of holding costs over 1,100 kg. The fuel above alternate plus final reserve must be known before accepting any hold. If holding would leave less than final reserve at the diversion aerodrome, no hold is available at all.
Weather. A 35–45 kt gust spread with a CB overhead and an active gust front on final is the classic microburst and windshear accident sequence. RVR 700 m is above company minima of 550 m, but minima compliance is irrelevant if a windshear encounter is likely: a go-around from low level in a microburst has killed crews who were legally entitled to start the approach.
Decision framework. (1) Obtain the latest ATIS/METAR and ask ATC for the RVR trend and windshear reports. (2) If a gust front is crossing or the CB is over the field, do not commence the approach. (3) Divert to OEDF while fuel comfortably protects alternate plus final reserve - conservative and defensible. (4) If attempting, set a hard commitment point (for example 1,000 ft AGL with all stable-approach criteria met) and go around without hesitation. (5) Notify ATC and operations control either way.
The examiner's point: having an alternate does not mean the destination must be attempted first. The alternate exists to be used.
3. With GNSS interference reported, how do you manage navigation and communication for the arrival?
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Separate the two problems: navigation integrity and the approach itself.
Navigation. Interference within 60 nm means FMS position and the RNAV path are not guaranteed. Notify ATC immediately in plain language - unable RNAV due GNSS interference - and request a conventional arrival: radar vectors to the ILS, or a VOR/DME procedure. Continuing to fly an RNAV STAR or RNP AR procedure while integrity is in doubt is not acceptable, because the required monitoring (position accuracy, integrity, ANP within limit) cannot be satisfied. Check the FMS navigation performance pages: a large ANP, repeated position shifts or an unexplained map shift confirms the interference.
Approach. The ILS is ground-based, so the localiser and glideslope are unaffected by jamming. The DME may be affected if it is GNSS-derived, and the FMS will not sequence an RNAV missed approach reliably. Brief a conventional missed approach: climb on the ILS or runway track to the published altitude, then expect radar vectors. Fly raw-data localiser and glideslope if flight-director guidance is degraded, and cross-check with the standby instruments.
Communication. Expect a congested frequency. Early, short notification is what lets ATC provide separation by other means. If ATC cannot offer a conventional approach and the weather is as forecast, a diversion to OEDF is the correct decision - not an improvised GNSS approach in dust with a CB overhead.