Smoke from huge wildfires near Sydney wafted over the tarmac as Jonas Doherty readied a newly modified Boeing 737 equipped to drop 4,000 gallons of firefighting liquid above the blaze. Mr. Doherty and his co-pilot descended to 150 feet and slowed the aircraft to about 150 miles an hour to empty the load, pressing a red button in the cockpit labeled “tank drop.” The duo and other air-tanker pilots are working 12-hour shifts and flying as many as 15 times a day as Australia battles its worst…