Night Spraying with a Failing GPS and Rising Terrain
You are contracted to spray a 60 ha block of maize at night, starting at 23:00. The block is rectangular, 1,000 m × 600 m, oriented east-west, with terrain rising 40 m from the west boundary to the east boundary. Your GPS guidance is showing intermittent position loss and a 15 m offset compared with the field boundary visible on the moving map; the receiver has been doing this for two loads. There is a 12 m high power line along the southern boundary and a farmhouse 200 m north of the block. Wind is 8 km/h from the west, temperature 18 °C, dew point 14 °C. You have a hand-held GPS as backup and a marked map. The client wants the job finished tonight because rain is forecast tomorrow. Fuel allows two more loads of 350 L each, and each load covers about 20 ha at the planned 17.5 L/ha.
1. Assess the navigation and guidance situation. Can you continue safely, and what are the risks?
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The GPS is not reliable for guidance. Intermittent position loss plus a 15 m offset against a visible boundary means the system may be wrong by more than the swath width in places, which is unacceptable for a 17.5 L/ha application where overlap and gaps both matter. A 15 m offset on a 18-20 m swath could produce double application on one pass and a missed strip on the next, with crop damage and product waste.
The specific hazards are: the 12 m power line on the southern boundary, where a 15 m lateral error could put the aircraft or the spray plume in the wrong place relative to the wires; the farmhouse 200 m north, which requires a buffer and a positive downwind check; and the rising terrain to the east, where a constant indicated height above the western datum would put the aircraft progressively lower over the crop, risking a strike on the eastern boundary or a late pull-up.
Continuing on the failing GPS alone is not acceptable. Options are to stop and troubleshoot (antenna, cable, satellite visibility, receiver fault), to switch to a verified backup guidance method (hand-held GPS with waypoints plus marked map and visual references), or to defer. The client's rain forecast is commercial pressure, not a safety argument.
2. If you continue with the backup method, how would you plan and fly the block to manage terrain, the power line and the farmhouse?
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Plan the block explicitly rather than relying on the failed unit. Using the hand-held GPS and the marked map, set waypoints at the four corners and at the power-line crossing points, and confirm the position against the visible field boundary before the first pass.
Terrain: because the ground rises 40 m west to east, fly the passes so that height above the crop is maintained by reference to the terrain, not a constant altitude. A practical method is to fly north-south passes, starting at the eastern (high) end and working west, so that each pass is over ground that is level across the swath and the aircraft is not climbing into rising terrain at the end of a run. Set a minimum height above crop and add a margin for the eastern boundary.
Power line: treat the southern boundary as a no-go line with a buffer equal to at least the product label buffer and never less than a safe lateral distance. Fly the southernmost pass so the aircraft and plume stay well north of the wires, and brief the exact turn direction away from the line.
Farmhouse: maintain the label buffer (200 m is the available distance, so check the label requirement) and avoid spraying when the wind would carry drift toward it. Wind from the west means drift is toward the east, roughly parallel to the block, which is favourable, but gusts and turns can change that.
Brief a go-around for every pass and a diversion to the nearest clear area.
3. What operational and human-factors considerations apply to finishing the job tonight?
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Fatigue: night work after two loads with a failing system increases workload and error rates. The pilot should assess cumulative duty time, rest breaks and whether the remaining two loads can be flown with full attention. If fatigue is significant, stop.
Workload: flying a precise swath at night with a hand-held GPS, a map and visual references is much more demanding than with a working moving map. Reduce task load by simplifying the pattern, using fewer, longer passes, and having a ground crew member with a radio confirm boundary positions and the power line.
Lighting: confirm the aircraft's night lighting, the field lighting or markers, and the ability to see wires and boundaries. The power line should be marked or illuminated if possible.
Decision-making: the rain forecast is a commercial pressure. The correct framework is to separate the safety decision (can this be flown accurately and safely?) from the commercial one (the client wants it done). If the guidance cannot be made reliable and the backup method cannot be verified before the next load, the job should be suspended and rescheduled, with the client informed. Record the GPS discrepancy and the decision in the operational log.