Bridge Closure and the Radio Net
A four-person patrol must collect a sealed sample from Site K and then report on the command radio net, which opens only at 15:00 and 17:00, each window lasting ten minutes. Base to Site K is 42 km at an average of 28 km/h, and the route crosses a bridge at km 30 that is closed for deck repairs from 12:00 to 14:00 daily. A secondary route via the ford adds 22 km and 40 minutes but bypasses the bridge entirely. Site K to Checkpoint 7 is 15 km at 20 km/h. Collecting the sample takes 45 minutes, and the radio needs 10 minutes of set-up before a window opens. The vehicle is serviceable and fuelled, and depart from base whenever you choose.
1. What is the latest departure time from base that still gets the sample and makes the 15:00 net? Show the backward calculation and explain why the obvious answer is wrong.
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The obvious answer (wrong): work back from 14:50 (10 minutes before the window) and subtract 45 min at K, 45 min K to Checkpoint 7, and 90 min base to K - a latest departure of 11:50.
Why it fails: departing 11:50 puts the vehicle at the bridge 30/28 h = 64 minutes later, at 12:54, which is inside the 12:00-14:00 closure. The patrol then sits until 14:00, reaches Site K at 14:26, leaves at 15:11 and misses the window altogether.
The correct calculation - work back from the binding constraint, the bridge:
- Clear the bridge by 12:00. Base to bridge = 30/28 h = 64 min, so depart base by 10:56.
- Site K: 10:56 + 90 min = 12:26.
- Sample complete: 12:26 + 45 min = 13:11.
- Checkpoint 7: 13:11 + 45 min = 13:56.
- Set-up complete 14:06, net opens 15:00 - 54 minutes of slack.
The constraint that binds is the bridge, not the drive. Plan to depart 10:45, not 10:56, to absorb a puncture or a slow gate.
2. The vehicle will not start and the fault takes 30 minutes to fix, so departure becomes 11:26. Compare the two options and state which you take.
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Option 1 - direct route, wait for the bridge. Bridge reached 11:26 + 64 min = 12:30; wait until 14:00 (90 minutes static); Site K at 14:26; sample complete 15:11; Checkpoint 7 at 15:56. The 15:00 window is lost; set-up can be done for 17:00, so reporting is two hours late.
Option 2 - ford. Base to Site K becomes 90 + 40 = 130 min, so Site K at 13:36; sample complete 14:21; Checkpoint 7 at 15:06. The 15:00 window is missed by six minutes - set-up alone needs ten.
Decision: take the ford.
- Both options miss the 15:00 net, so the net cannot be the tie-breaker.
- The ford delivers the sample to Checkpoint 7 fifty minutes earlier (15:06 against 15:56) and keeps the 17:00 report comfortable rather than marginal.
- It removes a 90-minute static wait at a known choke point - a stationary vehicle with four people and a sealed sample is the most exposed part of the whole route.
- It keeps the patrol moving in daylight and leaves the option of reporting position by voice on the secondary channel once in range.
Action: inform net control now that the 15:00 window will be missed and the report will be at 17:00. Do not gamble on a six-minute margin by cutting set-up or speeding - a missed radio check is a delay; a vehicle incident at the ford is a casualty.
3. In the original plan you have 54 minutes of slack. What do you do with it, and what decision point do you set?
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Do not spend it by leaving later. The slack exists because the bridge, not the drive, sets the departure time. Leaving later converts free time into a hard failure risk with no gain.
Use it where it buys safety:
- At Site K: reseal and label the sample properly, take a duplicate if the kit allows, record the seal number.
- At Checkpoint 7: by 14:06 you can test the radio on the secondary channel and pass an early position report, so the 15:00 window is a confirmation rather than a first attempt.
Decision point - 11:50 at the bridge. If the vehicle is not at the bridge by 11:50, the direct route cannot work; take the ford. Fixing the trigger in advance means the decision is made calmly, not under pressure at a closed barrier with a sample in the back.
Principle: schedule against the least flexible constraint and hold the slack as insurance, not as a later start time.