Now playing
03:43
Boeing never tested the failure of critical sensor
PHOTO: Courtesy AirlingRatings.com
Now playing
03:06
Virgin Atlantic CEO: We support Covid vaccine passports
Hayden A. Smith, 17, is a plane spotter and photographer and took this photo of UAL Flight 328 on Saturday, February 20, as it flew overhead in Aurora, Colorado.
PHOTO: Hayden A. Smith
Hayden A. Smith, 17, is a plane spotter and photographer and took this photo of UAL Flight 328 on Saturday, February 20, as it flew overhead in Aurora, Colorado.
Now playing
02:51
Dozens of Boeing 777 planes grounded after engine failure
ZEROe Airbus zero-emission concept aircraft
PHOTO: Airbus
ZEROe Airbus zero-emission concept aircraft
Now playing
04:51
Airbus CEO: Could ramp up production by second half of 2021
Now playing
02:27
Delta CEO: A negative Covid test mandate is 'a horrible idea'
VICTORVILLE, CA - MARCH 24: A Delta Air Lines jet taxis to be parked with a growing number of jets at Southern California Logistics Airport (SCLA) on March 24, 2020 in Victorville, California. As the coronavirus pandemic grows, exponentially increasing travel restrictions and the numbers of people in quarantine, airlines around the world are scrambling to find places to park a majority of their fleet as they wait to see how the situation will play out. (Photo by David McNew/Getty Images)
PHOTO: David McNew/Getty Images
VICTORVILLE, CA - MARCH 24: A Delta Air Lines jet taxis to be parked with a growing number of jets at Southern California Logistics Airport (SCLA) on March 24, 2020 in Victorville, California. As the coronavirus pandemic grows, exponentially increasing travel restrictions and the numbers of people in quarantine, airlines around the world are scrambling to find places to park a majority of their fleet as they wait to see how the situation will play out. (Photo by David McNew/Getty Images)
Now playing
04:37
Delta to block middle seats through April
PHOTO: CNN
Now playing
02:39
Commercial pilots blame pandemic downtime for in-flight mistakes
MARANA, ARIZONA - MAY 16: Decommissioned and suspended jetBlue commercial aircrafts are seen stored in Pinal Airpark on May 16, 2020 in Marana, Arizona.  Pinal Airpark is the largest commercial aircraft storage facility in the world, currently holding increased numbers of aircraft in response to the coronavirus COVID-19 pandemic.   (Photo by Christian Petersen/Getty Images)
PHOTO: Christian Petersen/Getty Images
MARANA, ARIZONA - MAY 16: Decommissioned and suspended jetBlue commercial aircrafts are seen stored in Pinal Airpark on May 16, 2020 in Marana, Arizona. Pinal Airpark is the largest commercial aircraft storage facility in the world, currently holding increased numbers of aircraft in response to the coronavirus COVID-19 pandemic. (Photo by Christian Petersen/Getty Images)
Now playing
05:24
JetBlue unveils new 'mint' business class suites
Passengers look out at American Airlines flight 718, a Boeing 737 Max, parked at its gate at Miami International Airport as people load for the flight to New York on December 29, 2020 in Miami, Florida. The Boeing 737 Max flew its first commercial flight since the aircraft was allowed to return to service nearly two years after being grounded worldwide following a pair of separate crashes. (Photo by Joe Raedle/Getty Images)
PHOTO: Joe Raedle/Getty Images
Passengers look out at American Airlines flight 718, a Boeing 737 Max, parked at its gate at Miami International Airport as people load for the flight to New York on December 29, 2020 in Miami, Florida. The Boeing 737 Max flew its first commercial flight since the aircraft was allowed to return to service nearly two years after being grounded worldwide following a pair of separate crashes. (Photo by Joe Raedle/Getty Images)
Now playing
03:15
Airlines & TSA boost security ahead of Inauguration
PHOTO: Courtesy AirlingRatings.com
Now playing
04:17
Etihad CEO: Targeting a complete turnaround by 2023
MIAMI, FLORIDA - DECEMBER 29: American Airlines flight 718, a Boeing 737 Max, takes off from Miami International Airport to New York on December 29, 2020 in Miami, Florida. The Boeing 737 Max flew its first commercial flight since the aircraft was allowed to return to service nearly two years after being grounded worldwide following a pair of separate crashes. (Photo by Joe Raedle/Getty Images)
PHOTO: Joe Raedle/Getty Images
MIAMI, FLORIDA - DECEMBER 29: American Airlines flight 718, a Boeing 737 Max, takes off from Miami International Airport to New York on December 29, 2020 in Miami, Florida. The Boeing 737 Max flew its first commercial flight since the aircraft was allowed to return to service nearly two years after being grounded worldwide following a pair of separate crashes. (Photo by Joe Raedle/Getty Images)
Now playing
02:38
Boeing's 737 Max returns to commercial service
Now playing
01:06
See Alaska Airlines' Covid-19 'Safety Dance'
PHOTO: WHDH
Now playing
02:46
Millions travel for Thanksgiving despite Covid-19 surge
Now playing
01:56
Emotional flight attendants say farewell after being furloughed
Now playing
04:40
Delta CEO: Rapid testing is critical to avoid quarantines
A United Airlines Boeing 777 aircraft waits to take off at Beijing airport on July 25, 2018. - Beijing hailed "positive steps" as major US airlines and Hong Kong's flag carrier moved to comply on July 25 with its demand to list Taiwan as part of China, sparking anger on the island. (Photo by GREG BAKER / AFP)        (Photo credit should read GREG BAKER/AFP via Getty Images)
PHOTO: Greg Baker/AFP/Getty Images
A United Airlines Boeing 777 aircraft waits to take off at Beijing airport on July 25, 2018. - Beijing hailed "positive steps" as major US airlines and Hong Kong's flag carrier moved to comply on July 25 with its demand to list Taiwan as part of China, sparking anger on the island. (Photo by GREG BAKER / AFP) (Photo credit should read GREG BAKER/AFP via Getty Images)
Now playing
03:09
United Airlines Chair: What the airline industry is facing is dire
(CNN Business) —  

An alert system that was supposed to be a standard feature on Boeing’s 737 Max aircraft “was not operable on all airplanes,” the company has said.

Boeing said in a statement Monday that the function wasn’t working on some of its planes because it was mistakenly linked to an optional feature, the angle of attack (AOA) indicator.

“The disagree alert was intended to be a standard, standalone feature on MAX airplanes. However, the disagree alert was not operable on all airplanes because the feature was not activated as intended,” it said. “Unless an airline opted for the angle of attack indicator, the disagree alert was not operable.”

The AOA indicator lets pilots know whether one of the AOA sensors is not working. The “disagree alert” shows if the sensors contradict each other. Boeing said the alert function was not necessary for the safe operation of the airplane.

Boeing (BA) has been battling rising public concern and pressure from shareholders over the safety of its best-selling jet after two of the planes crashed in six months. In October, a Lion Air plane crashed in Indonesia, killing all 189 people on board. An Ethiopian Airlines plane crashed under similar circumstances in March, leaving 157 people dead.

A Boeing 737 Max 8 airliner lifts off for its first flight in 2016 in Renton, Washington.
PHOTO: Stephen Brashear/Getty Images
A Boeing 737 Max 8 airliner lifts off for its first flight in 2016 in Renton, Washington.

Some of Boeing’s customers say the company wasn’t as transparent as it should have been about the alert system.

Southwest Airlines (LUV), the largest US operator of 737 Max planes, said in a statement on Monday it had only learned that the system was optional after the Lion Air tragedy in October.

“Prior to the Lion Air event, the AOA disagree lights were depicted as operable (not optional) by Boeing on all Max aircraft,” Southwest spokeswoman Michelle Agnew said.

Boeing defended its practices on Monday.

“On every airplane delivered to our customers, including the Max, all flight data and information needed to safely operate the aircraft is provided in the flight deck and on the flight deck display,” it said.

But the statement marks the first time the company has admitted a mistake in the rollout of its 737 Max.

At its shareholder meeting earlier on Monday, Chief Executive Dennis Muilenburg said that the safety systems on the 737 Max were properly designed.

“We haven’t seen a technical slip or gaffe in terms of the fundamental design and certification of the approach,” he said.

He also suggested that in both fatal crashes, pilots did not “completely” follow the procedures that Boeing had outlined to prevent malfunction.

Boeing declined to comment further on the matter.

The company has said it will continue working to make its planes safer. Last month it deployed a software overhaul for the 737 Max aircraft that involved an upgrade to MCAS, the anti-stall system that has been identified as a common link in both crashes.

The most significant change to the MCAS software in that update is the addition of data from a second AOA sensor, which measures the horizontal tilt of the airplane. In its first iteration, the software only drew data from one AOA sensor.

Investigators in Indonesia have said that faulty data from a single sensor caused the MCAS system to repeatedly push the Lion Air flight down towards the ocean, in a battle with the pilots who tried to right the plane.

Boeing says it will ensure that the “disagree alert” system is operable on all 737 Max planes.

Greg Wallace, Chris Isidore and David Shortell contributed to this report.