Nobody Left to Fly Christmas — Python Bug Hunt

Briefing

Modelled on American Airlines, November 2017. A glitch in the airline's pilot scheduling system let pilots drop trips over the December holiday period, and thousands of flights — reportedly around 15,000 — ended up without a pilot assigned. The airline said it would pay pilots extra to pick the trips back up. The precise root cause was not published.

This reconstruction models the failure as a trip-drop request that is approved whenever the pilot is on the trip, without checking what the trip is left with.

Fix request_drop so no approval leaves a trip under its minimum crew.

Bug report

BUG-CREW-1712 · Priority: Critical · Reported by: crew scheduling

request_drop(schedule, trip_id, pilot) returns True and removes the pilot from schedule[trip_id]["assigned"] ONLY if:

• the trip exists
• the pilot is currently assigned to it
• after removal the trip still has at least rules.min_crew(trip) pilots

Otherwise it returns False and leaves the schedule unchanged.

Observed: drops on fully-staffed December trips were approved, leaving those trips with fewer pilots than they need.

Logs

[crew] drop approved trip=AA1217 pilot=P-4410 assigned=1 required=2
[crew] drop approved trip=AA1217 pilot=P-2981 assigned=0 required=2
[crew] 2017-12 open trips without crew: rising

The code as shipped

src/crew/drops.py (editable)

rules = bug_require("./rules.py")


def request_drop(schedule, trip_id, pilot):
    trip = schedule.get(trip_id)
    if trip is None:
        return False
    if pilot not in trip["assigned"]:
        return False
    trip["assigned"].remove(pilot)
    return True

Read-only context: src/crew/rules.py.