On the morning of January 26, as two Alaska Airlines flights departed from Seattle to Hawaii six minutes apart, the pilots each felt a slight jolt and the flight attendants in the rear of the cabin heard a scraping noise.
When the noses of both Boeing 737s lifted into the sky during takeoff, their tails had scraped the runway.
Both planes immediately circled back and landed back at Seattle-Tacoma International Airport. Tail strikes happen occasionally in aviation, but two in quick succession was not normal.
Bret Peyton, Alaska’s acting operations manager, immediately ordered no more aircraft to operate on the airline’s route network. All Alaska flights that were not yet airborne have been halted statewide.
“At that point, like two in a row, I was like, ‘No, we’re done,'” Peyton said. “I stopped there.”
For Peyton, who was a lieutenant colonel in the Air Force, that crucial call was a heartbreaking moment. But few travelers, aside from passengers aboard the two Hawaii flights, who had to wait several hours to continue their journey, would have noticed anything unusual.
The standstill lasted only 22 minutes.
Alaska flight operations staff quickly discovered that a software bug was sending erroneous takeoff weight data to crews. They immediately found a workaround and normal flying resumed.
Last Tuesday, following a series of safety incidents and dangerous near misses in the US aviation system, Acting Administrator of the Federal Aviation Administration Billy Nolen wrote a “call to action” letter warning that the US system’s excellent safety record should not be taken for granted.
The January 26 stern attacks on Sea-Tac were not close calls; The passengers on these Hawaii flights were never in danger. Still, the glitches point to the need for pilots to be more vigilant when reviewing automated data.
“We rely on this data to safely operate the plane,” said an Alaska Airlines captain who flew a 737 to Hawaii and requested anonymity for speaking without the company’s permission.
However, the incidents also offer some reassurance, as Alaska promptly shut down service until the cause was understood and addressed.
“Alaska took care of it very quickly and appropriately,” the captain said.
£20,000 error
The first incident occurred when Alaska Flight 801, a Boeing Max 9 bound for Hawaii’s Big Island, took off at 8:48 am
At 8:54 am, Alaska Flight 887 followed, this time a Boeing 737-900ER bound for Honolulu.
To determine takeoff thrust and speed settings, Alaska’s pilots and others use a performance calculation tool provided by a Swedish company called DynamicSource.
It delivers a message to the cockpit with key weight and balance data, including the number of people on board, the jet’s empty and gross weight, and the location of its center of gravity.
In a pre-launch cockpit check, this data is fed into the flight computer to determine how much thrust the engines will deliver and at what speed the jet will take off.
An American Airlines pilot using the same DynamicSource performance data tool, who also spoke anonymously because he didn’t have permission, explained that the computer then calculates just the right amount of engine thrust so pilots don’t use more than necessary.
“The goal is to lower the power consumed at takeoff,” he said. “This reduces engine wear and saves money” on fuel and maintenance.
Flights to Hawaii are usually full, with lots of baggage and a full load of fuel for the journey across the ocean. The planes are heavy.
This morning, a software bug in an update to the DynamicSource tool caused it to return seriously underestimated weights for the planes.
The Alaska 737 captain said the data was in the range of 20,000 to 30,000 pounds of light. With these jets weighing between 150,000 and 170,000 pounds combined, the error was enough to skew engine thrust and speed settings.
Both aircraft flew over the runway with less power and speed than they should have. And since the jets were judged to be lighter than they actually were, the pilots turned early.
Both the Max 9 and 737-900ER have long passenger cabins, making them more prone to tail strike if the nose comes up too early.
Alaska says it operated 727 flights that day, only 30 of which had incorrect takeoff dates. Only these two planes flying to Hawaii had tail strikes.
Then, Alaska Air Operations officials and safety experts from the Air Line Pilots Association independently analyzed data from the two flights to assess the safety risk. Everyone noted that despite the reduced thrust, both aircraft got airborne within safety limits.
The data “confirms that the aircraft was safely airborne with the runway remaining and was at the end of the runway at an altitude well within prescribed safety margins,” said union Alaska unit chairman Will McQuillen, in an explanation.
The fuselage under the tail of a jet has a bump called a “tail skid” that serves to crumple and absorb shock. Still, maintenance technicians are required to inspect the damage, so the two planes immediately returned to the airport.
Both aircraft were cleared to fly again later that day. In fact, the Max 9 was cleared in time to take off at 12:30 p.m. to fly the passengers who had disembarked that morning to Kailua-Kona.
“That looks about right”
The error was quickly identified in part because some flight crews noticed that the weights did not appear to be correct and requested manual validation of the numbers.
When the DynamicSource message is received during pre-flight inspection, the first officer reads each data point aloud and the captain verbally acknowledges each.
Shortly after the tail strike that day, Alaska issued a “safety blitz” message to all of its pilots, advising them to “take a second and do a data sanity check” when entering the DynamicSource information.
In other words, they should pause when the weights seem to be decreasing.
The Alaska captain said that, as with many things in aviation, pilots routinely use an acronym when conducting the pre-takeoff “sanity check”: TLAR, which means “This looks about right.”
If the automatically loaded data does not seem correct to any of the pilots, they can make a manual request for take-off data from the Airline Operations Center. “But 99.8% of the time, the data is correct,” he said.
Peyton of Alaska said, “Several crews have noticed the error and notified shipping.”
The pilot at American Airlines said “requesting manual data is not standard” and that if there is a glitch, of course some pilot somewhere will miss it.
“Not everyone gets eight hours of sleep the night before. Someone is going through a divorce. Someone’s not that perceptive this morning,” he said. “The health check is not perfect every day of the week.”
To pull the plug
After Peyton called the outage that morning, the discrepancy in the DynamicSource weight data became apparent.
“This discovery happened in a very short period of time at about 8:45 a.m.,” he said. “It all happened very, very quickly, as did the airline closure.”
A quick interim solution turned out to be simple: When the operating staff switched off the automatic uplink of the data to the aircraft and switched to manual queries, “we no longer had the error.”
Peyton said his team also verified the integrity of the calculation itself before the suspension was lifted. It was all done in 20 minutes.
About five hours later, the software code was permanently repaired.
Peyton added that although the DynamicSource software update was tested over a period of time, the bug was overlooked because it only appeared when many aircraft were using the system at the same time.
A test of the software was then developed under high demand.
Peyton said his first call that day was to the airline’s chief dispatcher to halt operations. His second was with the FAA to let the agency know what was going on.
Tuesday’s warning letter from Acting FAA Administrator Nolen was spurred by a string of recent airline incidents that narrowly avoided becoming fatal accidents.
Adding to several runway raids, the sharp dive toward the ocean by a 777 departing from Hawaii in December, and this month’s close encounter between a FedEx 767 coming in to land and a Southwest Airlines 737 departing from the same Austin runway took off, Texas, raised a special alarm.
14 years have passed since the last fatal US plane crash. There is concern that less experienced pilots and air traffic controllers hired during the post-pandemic labor shortage could reduce safety margins.
Nolen said he ordered a security review “to examine the structure, culture, processes, systems and integration of the security efforts of the US aerospace system.”
And he called a summit in March to determine “what additional actions the aviation community needs to take to maintain our safety record.”
FAA spokesman Ian Gregor said Thursday the agency is investigating the incidents in Alaska. He confirmed the airline’s report that the planes took off within safety parameters.
Peyton said the airline’s leadership was very supportive of his decision to pull the plug that January morning.
“We had to stop operations. It was very clear to me in a very short time and I’m glad we did that,” he said.
“I didn’t go to work that morning because I thought I was going to stop a major airline,” Peyton added. “It tells me that I am authorized to do this and so is every employee here. That is part of our safety culture.”