Last week, a Delta Air Lines 
Upon arriving back in St Louis, the aircraft involved, a 25-year-old 717 that bears the registration N935AT, stayed on the ground for two days before repositioning to Atlanta. It reentered service shortly afterward, but has since been involved in another diversion while operating a scheduled domestic flight out of 
What Happened?
Delta Air Lines flight DL9962 was a service that originated at Lambert St Louis International Airport (STL) in the US federal state of Missouri. Its destination was Detroit Wayne County (DTW) in Michigan, with Delta serving this 90-minute route four times a day in each direction. Flightradar24 suggests that DL9962 was a repositioning flight, as this number is not normally used on this route, and there were just two pilots on board.
On December 2, this flight took to the skies at 10:59 am, and was scheduled to reach Detroit just over an hour and a half later, at 12:26 pm. However, by 11:12 am, it was back on the ground, with the Aviation Herald reporting that the pilots stopped the climb at 5,000 feet due to an acidic smell in the cabin. After landing, the aircraft stopped on the runway for five minutes before continuing. A Delta spokesperson told Simple Flying:
“On Tuesday, December 2, shortly after takeoff, DL9962 from St Louis to Detroit returned to STL out of an abundance of caution after an odor was detected in the cabin. Our flight crew followed established procedures and landed safely.”
The Aircraft Has Since Experienced Another Diversion
Reporting by the Aviation Herald noted that the runway that the Boeing 717-200 involved in the diversion was the same one that it had taken off from, namely the 2,743-meter (9,000 feet) concrete strip denoted by the heading 29. The reason for the need to reposition the aircraft from St Louis to Detroit without passengers came after the cancellation of a scheduled passenger service on this route the previous day, namely flight DL1626.
After N935AT’s unplanned diversion back to St Louis, it was originally planned that the aircraft would instead be repositioned to Atlanta (ATL) on the evening of December 3, with Flightradar24 noting that it was meant to do so as DL9974 at 10:30 pm. However, the rear-engined twinjet actually didn’t end up leaving until 2:40 pm on December 4, meaning that it spent more than two days on the ground. It eventually arrived in Atlanta at 4:50 pm.
After this, it operated a brief circular test flight out of Atlanta on the evening of December 5, before reentering service the next day with a round trip to Memphis (MEM) and back to Atlanta. Yesterday morning, it was scheduled to fly to Huntsville at 9:09 am local time as DL1708, but, after departing more than three hours late at 12:33 pm, this flight also had to divert back to its point of origin, touching back down in Atlanta at 1:05 pm local time.
Smoke Odor Prompts Delta Air Lines Boeing 717 Diversion To Chicago O’Hare
Delta Air Lines moved the affected Boeing 717 to Minneapolis-Saint Paul Airport shortly after the incident.
Delta’s Boeing 717 Fleet In A Nutshell & Other Similar Incidents
As previously noted, the Boeing 717 involved in last week’s smell-related diversion bears the registration N935AT. According to present fleet data made available by ch-aviation, this jet is 25.72 years old, and one of 80 717s in the Delta fleet. Data from aeroLOPA shows that it has 12 first and 98 economy seats.
This isn’t the first time in recent memory that a US airliner has had to divert due to an unusual smell in the cabin. Another similar incident occurred last week involving a United Airlines Boeing 777-200ER flying from London (LHR) to San Francisco (SFO) that had to divert to Edinburgh (EDI). Elsewhere, in March of this year, a Southwest 737 flying to Louisville had to return to Atlanta, with a burning smell being disclosed as the reason.