The Four-Checkpoint Route
You are the section commander of a six-person cadet section preparing a dismounted route that must pass through four checkpoints, A to D, in sequence, to validate a navigation qualification. The legs are: A to B, 4.2 km on firm trail; B to C, 3.0 km including 250 m of ascent; C to D, 5.5 km on a vehicle track. Planning speeds are 4 km/h on trail, 3 km/h on the climbing leg and 5 km/h on the track. Two 10-minute administrative halts are mandatory. Your slowest member carries 22 kg, and the section's overall movement time must be increased by 15 per cent to account for him. You step off at 13:15 and daylight ends at 18:40. A flooded ford on the B to C leg forces a detour that adds 40 minutes. Each person carries 2 litres of water; consumption in the forecast 31 °C is 0.6 litres per hour. The section radio was switched on at 13:00 and its battery is rated for five hours. No resupply is available before checkpoint D.
1. Compute the earliest arrival time at checkpoint D and state the daylight margin. Show each step.
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Base movement time (before the 15 per cent factor)
- A–B: 4.2 ÷ 4 = 1.05 h = 63 min
- B–C: 3.0 ÷ 3 = 1.00 h = 60 min, plus the 40 min detour = 100 min
- C–D: 5.5 ÷ 5 = 1.10 h = 66 min
- Sum: 63 + 100 + 66 = 229 min
Apply the slowest-member factor: 229 × 1.15 = 263.4, round up to 264 min of movement. Add halts: 264 + 20 = 284 min = 4 h 44 min. Arrival: 13:15 + 4:44 = 17:59. Daylight ends 18:40, so the margin is 41 minutes.
Two cautions belong in the answer. First, that 41-minute margin is smaller than the 40-minute detour, so a second unplanned delay of the same size consumes the entire buffer. Second, the figure assumes the 15 per cent penalty covers the climbing leg as well; if it applies only to flat movement the arrival moves in your favour, so state the assumption explicitly rather than presenting 17:59 as certainty. The useful output of the calculation is not the clock time but the identification of which resources are consumed before 17:59.
2. Using the same figures, identify which resource fails first and support your answer with numbers.
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Test each constrained resource against the 4 h 44 min plan.
Water. Elapsed time from step-off is 4.73 h. Consumption at 0.6 L/h = 0.6 × 4.73 = 2.84 L per person against 2.0 L carried. Deficit ≈ 0.84 L each, roughly 5 L across the section. Water runs out at 2.0 ÷ 0.6 = 3.33 h after 13:15, i.e. about 16:35 - nearly an hour and a half before arrival, and before the hottest part of the afternoon ends.
Radio. Switched on at 13:00, rated five hours, so it dies at 18:00, one minute after the planned arrival. Any delay at all means the final leg and the arrival report are made with a dead set.
Daylight. 41 minutes of margin - adequate.
So water binds first and hardest, radio second, daylight not at all. An answer that says 'it is tight but feasible' without separating these three has not done the analysis: the plan is feasible on daylight and infeasible on water, which are different conclusions requiring different fixes.
3. Recommend an amended plan. Which single change buys the most, and what do you tell higher headquarters before step-off?
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Highest-value change: cache water forward. Put 1 litre per person (6 L, plus 2 L spare) at checkpoint C by vehicle before 13:15. This closes the 0.84 L deficit without adding a single minute of walking, which is better than any change that spends time to save water. The alternative - a 15-minute resupply halt at C - also works but pushes arrival to about 18:14 and cuts the daylight margin to 26 minutes.
Second fix: carry a charged spare battery and swap at B (about 2 minutes) or at C, and keep transmissions short so the set is alive at the finish for the arrival report.
Contingency: shorten one halt to 5 minutes, and set a decision gate - checkpoint B by 14:30, checkpoint C by 15:55. Miss the gate and the section cuts the C–D leg to a direct track route or turns back, decided in advance rather than argued about in the dark.
Report to higher before step-off: route, timings for each checkpoint, the detour, the water cache request, the spare battery, and the gate times. A plan that depends on a cache you never requested is not a plan.