SPECI, Wind Shear and a Falling QNH at Las Américas
At 1930Z a line of thunderstorms from a tropical wave lies 15 nm west of Las Américas (SDQ) and is moving east at 20 kt. The 1925Z SPECI reads: wind 190/15G35 kt, visibility 1,500 m in heavy thunderstorm rain, SCT012CB, temperature 29 °C, dew point 25 °C, QNH 1001 hPa, with a TEMPO trend. The QNH two hours earlier was 1009 hPa. A preceding landing aircraft reported a 20 kt airspeed loss at 500 ft on the ILS RWY 17 and a 'very rough' final. Two arrivals are on approach, an A321 at 6 nm and a Dash 8 at 12 nm; a B737 is holding short of RWY 17; and an R44 helicopter is 5 nm south at 1,000 ft requesting to cross the control zone VFR. The aerodrome has no low-level windshear alerting system; pilot reports are your only source. Terrain west of the field rises to 3,500 ft.
1. Keep or suspend runway 17 operations? Show what the wind, the SPECI and the pilot report each tell you, and what you will do with the three aircraft you control.
Read the worked answer
Separate the three sources of evidence.
Wind: 190/15G35 against runway 170 is only 20° off, so it is a headwind of about 14 kt gusting 33 kt with only 5–12 kt of crosswind. Purely on wind components, RWY 17 is still the correct runway; a runway change is not justified yet.
SPECI: a CB at 1,200 ft over the field and 1,500 m visibility in heavy rain means convective conditions are now overhead, not forecast. The cell is 15 nm west and moving east at 20 kt, so it arrives in about 45 minutes.
Pilot report: a 20 kt loss at 500 ft is severe low-level wind shear. With no LLWAS, this report is your primary warning and must be passed to every arrival, added to the ATIS, and passed to the departing crew.
Actions: pass the shear report to the A321 and Dash 8 and let each crew decide; the A321 at 6 nm is committed, so brief it and expect a go-around. Offer the Dash 8 a delay vector or hold so the worst of the cell passes. Hold the B737 on the ground - a departure into that cell and shear is the most exposed option. Request an updated trend forecast and check for a SIGMET.
2. What do you do with the R44 request, and why?
Read the worked answer
Refuse the crossing and tell the pilot why.
The reported visibility at the field is 1,500 m in heavy thunderstorm rain. That is below the VMC minima for a control zone, regardless of the helicopter's height or the pilot's experience, so the flight cannot legally continue under VFR inside the control zone on that report. Add a CB at 1,200 ft, a surface wind gusting 35 kt and a severe wind shear report at 500 ft, and a light helicopter at 1,000 ft would be flying into the worst of the day's conditions.
Phraseology: 'R44, cross the control zone is not available, VFR flight not possible, reported visibility 1,500 metres in thunderstorm rain, advise intentions.' Offer what you can: a route clear of the control zone, or holding south of the zone until the cell passes, or landing at the nearest suitable site.
Do not be talked into a 'special VFR' style workaround. If the pilot elects to continue, say clearly that a clearance is not issued and that the flight is operating outside the clearance, then inform the supervisor. If the aircraft subsequently enters the zone, you handle it as an emergency but you do not retrospectively authorise it.
3. The Dash 8 reports its altimeter was set to 1013 hPa during the descent to 4,000 ft. What do you tell it, and how large is the error?
Read the worked answer
The current QNH is 1001 hPa, twelve hectopascals below the standard setting. At roughly 27 ft per hPa that is about 320 ft; using the 30 ft rule of thumb, about 360 ft. Either way, the aircraft is several hundred feet lower than its altimeter shows, because a setting that is too high makes the instrument over-read.
Tell the crew at once: 'Dash 8, altimeter setting 1001, QNH 1001, you were approximately 300 feet low.' Ask them to confirm the setting, pass the QNH again in the descent clearance, and confirm the field elevation check is consistent.
Why it matters here: terrain west of the field rises to 3,500 ft. An aircraft descending to 4,000 ft with a 300 ft error has only 200 ft of real margin over that terrain, which is why the 'high to low, look out below' rule is not academic. Also warn the next arrival before it sets its altimeter, and check that the transition level is still high enough above the transition altitude to keep 1,000 ft between the lowest usable flight level and the transition altitude; the QNH has fallen 8 hPa in two hours, so the transition level has moved up.