Cumulonimbus on track: Ardmore to Tauranga
You are flying a Cessna 172 from Ardmore to Tauranga at 5,500 ft on a February afternoon, 1530 NZDT. On track ahead, a line of cumulonimbus has built along the Kaimai Range. The top is estimated at FL280 and the base at 2,500 ft, with a clearly defined rain shaft and frequent lightning. Radar shows the cell is 12 km wide and moving southeast at 15 kt, almost along your track. A SIGMET is current for embedded cumulonimbus and severe turbulence below FL200. The Tauranga TAF carries PROB40 TEMPO 1500/1700 3000 +TSRA BKN015CB. You have 2 h 10 min of fuel, Tauranga is 25 minutes ahead, and Hamilton is 20 minutes behind you and reported clear. The aircraft has no weather radar, no de-icing and no oxygen.
1. List the hazards this cell presents to your flight, in order of threat, and how each would manifest in the cockpit.
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- Downburst and low-level wind shear. The visible rain shaft is the marker. A downburst can give a sudden 30-50 kt change in headwind component and a downdraught over 1,000 fpm near the surface, which is unrecoverable at low height in a 172.
- Severe turbulence. In and under the cell, and beneath the anvil, the updraughts and downdraughts can exceed the aircraft's design gust load; expect loss of control and possible structural damage rather than just an uncomfortable ride.
- Hail. It can be encountered several kilometres outside the visible core, carried downwind in the anvil, and can damage the leading edges and windscreen in seconds.
- Icing. Strong updraughts carry supercooled water high, so icing is possible in the cloud even in summer at your levels.
- Lightning. Airframe strikes, temporary blindness at night, and disruption of radios and magnetos.
- Visibility and cloud base. The rain shaft reduces visibility to near zero and the 2,500 ft base leaves no VFR room underneath; the practical effect is that any penetration is instrument flight in an aircraft and with a pilot not equipped for it.
- Ancillary errors. Pitot and static errors, and unreliable compass and gyros in heavy rain and turbulence.
2. Do you continue, deviate, or turn back? Show the numbers and the decision rule you are applying.
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Turn back to Hamilton now.
Numbers: the cell is 12 km wide and its top is FL280, so over-the-top is impossible at 5,500 ft and there is no under-the-base option because the rain shaft reaches the ground. Standard guidance is at least 20 km lateral clearance from the cell and its anvil. The cell is tracking southeast at 15 kt, almost along your track, so a deviation to the east or west must be large and must be completed before the cell reaches the point where you would cross its path.
Fuel: 130 minutes total, Hamilton 20 minutes behind, leaving roughly 105 minutes on the ground - comfortable. Tauranga is 25 minutes ahead plus at least 15-20 minutes for a wide deviation, and the destination carries PROB40 TEMPO for thunderstorm rain, reducing the chance of a VFR arrival with no good alternate.
The decision rule is simple: if you cannot keep at least 20 km from the cell and remain VMC with a clear escape route, you do not continue. The decision point must be crossed while you still have the ability to turn around, not when the cell is abreast of you. Never hold or orbit near a cumulonimbus to wait for it to move; the anvil and hail extend well beyond the visible core.
3. If you divert to Hamilton, what would you check before and during the diversion?
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Before turning, check that Hamilton's current METAR and trend actually support the decision rather than relying on the earlier report - cells develop quickly on a summer afternoon over the Waikato, and the diversion must not put you into a second cell. Check the area QNH and the minimum safe altitude for the diversion track, and confirm your position and fuel state against the new plan.
Operationally: broadcast or advise ATC of the diversion and your level, confirm the airspace and traffic along the route back, and note that other VFR traffic will be doing the same thing. Set a personal minima for the diversion - if Hamilton deteriorates below VMC criteria, you need a third option, so identify one (for example, a coastal field) before you need it.
On the way back, keep a continuous visual check on the cell's anvil downwind and on any new development, keep clear of the rain shaft, and keep your escape route behind you. After landing, get an updated forecast and do not launch again on the assumption that the cell will have moved - use the actual METAR, radar and the visual picture.