Ever since 9/11, when an airliner suddenly goes silent over American airspace, it triggers an immediate question: is the silence caused by a simple radio-out issue, or something FAR more sinister?

This is the story of Northwest Airlines Flight 188.

It’s October 21, 2009. Two senior pilots prepare for a routine flight from San Diego, California to Minneapolis, Minnesota. This trip normally takes just over 3 hours.

In the left seat sits a 53 year old veteran captain with over 18,000 flight hours. Almost half of that experience was gained from commanding aircraft similar to the A320. He’ll be ‘pilot flying’ for this trip.

The first officer, aged 54, brings a similar depth of experience, nearly 14,000 hours total, with over 5,300 specifically in this aircraft type. The first officer will be ‘pilot monitoring’.

As is reasonably common in large airlines, this was only the pilots’ second day working together as a crew. On board are 144 passengers, 3 flight attendants and the 2 pilots.

Their aircraft, an Airbus A320, is a popular workhorse for many airlines. Its sophisticated autopilot system and fly-by-wire controls are designed to enhance safety and reduce pilot workload. The plane has a robust communication suite including three separate VHF radios, each capable of independent operation, a redundancy meant to ensure constant contact with air traffic control. And THIS will become important later on.

So, while this was a Northwest Airlines flight, the airline was in the midst of a merger with Delta Air Lines. For important context, the merger, which created what was briefly the world’s largest airline, brought about complex challenges for all involved. Pilots from both carriers suddenly faced new standard operating procedures and different flight deck protocols. They had to adapt to new procedures every two months as the changes were gradually phased in. And among these changes was a new computerized flight crew scheduling system that would soon play a role as this story unfolds.

At 4:59 P.M., flight 188 had a normal take off from San Diego. It climbs to its cruising altitude of flight level three seven zero, or 37,000 feet as the autumn sun begins to set. The flight attendants serve food and drink to the passengers who have settled in for their flight.

As is often the case with Airbus flight crew, both pilots removed their headsets, opting to monitor air traffic control through the cockpit speakers. The autopilot dutifully guides the aircraft as it cruises along its pre-programmed route. All good so far.

It’s soon dinner time and with the arrival of the lead flight attendant onto the flight deck with meals for the pilots, the captain takes the opportunity to step away for a restroom break. Following security protocols, the flight attendant remains in the cockpit until the captain returns.

At 6:39 P.M., the first officer checked in with Denver Air Traffic Control. Around 6 minutes later at 6:45 P.M., the first Denver controller requested that Flight 188 contact the next Denver center which the first officer acknowledged. Less than a minute later, the first officer contacted the second Denver controller and checked in by reporting altitude at 37,000 feet. This was acknowledged by the second Denver Controller.

With the captain’s return to the cockpit, he and the first officer started discussing recent changes to their airline’s preferential bidding process. It’s during this time that the controller at Denver Center had to repeat a transmission to the aircraft. On the Denver controller’s second attempt to reach Flight 188 and have them switch over to the next Denver center on 132.17 MHz, the first officer responds, “okay three two one seven, northwest one eighty eight.” This standard readback by the first officer will become important later on, we’ll come back to it.

At 6:57 P.M., although the pilots didn’t know it yet, that would be the last radio transmission they’d make – for quite some time.

It wasn’t long before both pilots were absorbed in a deep discussion about Delta’s new Preferential Bidding System. The captain wasn’t happy with his allocated schedule for next month as it didn’t suit him for personal family reasons. Frustrated with that, the captain pulled out his personal laptop to show the first officer his schedule.

The first officer, attempting to help, took out his own laptop, and began tutoring his colleague on the intricacies of the new bidding system. He placed his laptop on his tray table and proceeded to explain what he understood of the new system. Unlike Boeing pilots who have to contend with control columns between their legs, the A320’s side-stick design offers the luxury of proper tray tables, though these are meant for meals, not impromptu computer sessions.

Now seems a good time to note that the use of personal electronic devices or P.E.D.s in flight was strictly forbidden. The rule wasn’t just about preventing distractions – laptops could block critical flight displays, and their batteries posed a fire risk. Neither pilot could have guessed how their simple rule violation would soon spiral into something much bigger.

Meanwhile, at 7:05 P.M., the third Denver sector tried to contact Flight 188 to advise them of the assigned frequency for the fourth Denver sector. No response. They repeated the transmission; again, no response. The aircraft entered the fourth Denver sector’s airspace, and later, the fifth, without making any radio check-ins.

Back on board, the pilots, still deep in conversation about the new scheduling system, seemed oblivious to the attempts to contact them. Then things escalated.

On the ground, air traffic controllers began their standard procedures for an aircraft that had gone “NORDO”, meaning no radio communications. Denver Center controllers attempted multiple radio calls.

By 7:24 P.M., Flight 188 had been radio silent for 27 minutes, quietly crossing through sector after sector of airspace. The fifth Denver controller, realizing something wasn’t right, called Minneapolis Center to check if the aircraft had made contact. When Minneapolis confirmed they hadn’t heard from Flight 188 either, Denver tried reaching Northwest’s dispatch office. But even this proved challenging as they were dialing an outdated number, inactive since a recent office relocation because of the merger.

Eventually, the controllers got through to Northwest’s dispatch office. Airlines could usually reach their pilots through ACARS text messages when radio contact failed. What nobody realized at the time was that while these messages were indeed reaching the aircraft, the pilots remained oblivious to the green “ACARS MSG” alerts silently appearing on their displays.

At 7:39 P.M., Northwest Flight 196 was asked to try reaching their silent colleagues. Back in dispatch, more ACARS messages were sent. Each attempt at contact, including the international emergency frequency 121.5 MHz, went unanswered, and tensions on the ground began to rise.

The Airbus A320’s sophisticated autopilot was faithfully following the programmed flight plan, hitting waypoint after waypoint. The cockpit’s ECAM display continued to show unacknowledged message alerts, while various radio frequencies carried increasingly urgent calls that went unheard through the flight deck speakers.

What was happening? Had both pilots fallen asleep? Or was there something more sinister at play?

At 8:01 P.M., the Flight Management Computer ran out of its pre-programmed waypoints and, as expected, silently switches from navigation to heading mode. On the primary flight displays, “NAV” changed to “HDG”. The aircraft does what it is told and continues northeast into the growing darkness, now flying a constant heading rather than its planned route.

Meanwhile, in Minneapolis Center, the increasingly concerned controllers were running out of options. Multiple attempts across different frequencies had failed. Other aircraft couldn’t reach the phantom aircraft. Company dispatch messages weren’t getting through.

At 8:03 P.M., control was transferred to the fourth Minneapolis controller, who would soon face the unsettling realization that this aircraft was about to overfly its destination.

Controllers had followed their procedures for possible “suspicious aircraft,” and preparations were being made for military intervention. The Minneapolis Operations Manager in Charge had already contacted the Domestic Events Network, and fighter aircraft were being readied for intercept. The situation room at the White House was informed.

On the flight deck, the conversation about crew scheduling continued, both pilots unaware of the building tension their silence was creating. Neither noticed when their navigation displays showed that they overflew Minneapolis-St. Paul International Airport, their intended destination. The autopilot, designed with multiple layers of pilot alerting, continued to function exactly as designed, but those alerts were only effective if pilots remained engaged with their primary duty: flying the aircraft.

Suddenly, a flight attendant called the cockpit, asking about their arrival time. Finally, this was the moment of realization for the pilots. With the spell of distraction now broken, the captain looked down at his navigation display and saw Duluth to his left and Eau Claire to his right. They had overflown their destination by over 100 miles!

It was at this time, 8:12 P.M., the Fourth Minneapolis controller made another attempt at contact. Simultaneously, Northwest Airlines dispatch sent yet another ACARS message.

Flight 188 suddenly broke its silence, but, on the wrong frequency of 132.125 MHz. This frequency was eerily close to the last one acknowledged by the pilots – 132.17 MHz. While the first officer’s earlier readback of the frequency was correct, it seems that he did not activate it correctly at the time, possibly due to the earlier distraction. In the A320 cockpit, changing frequencies requires two steps: using a rotary dial to first select the desired frequency, THEN pushing a transfer button to activate it. For whatever reason, it seems that this step wasn’t performed correctly.

The crew, who had inadvertently tuned to Canada’s Winnipeg Area Control Center’s frequency, told controllers they had overflown their destination and needed to turn around. The Canadian controllers, quickly establishing the aircraft’s position over Eau Claire, Wisconsin, redirected the pilots to the correct Minneapolis Center frequency.

When Flight 188 finally established contact with Minneapolis Center at 8:14 P.M., the pilot’s explanation was, let’s just say, a little understated: “Roger, we got distracted and we’ve overflown Minneapolis. We’re overhead Eau Claire and would like to make a one eighty and do arrival from Eau Claire.”

What the pilots weren’t aware of just yet was that this “distraction” had created serious security concerns 37,000 feet below them. During the 1 hour and 17 minutes of silence, the aircraft had triggered multiple layers of emergency response planning. And this will soon have significant consequences for the two pilots involved. More on that later.

The controllers who were still deeply concerned about potential security threats, needed to verify the cockpit was not compromised. The pilots assured them the flight deck was secure – there had been no security breach. As they approached Minneapolis, controllers, still uncertain about the situation, had the aircraft perform some specific flight maneuvers to verify the crew was indeed in full control of the plane. When pressed about what had caused the long radio silence, the pilots would only say they had been distracted by company issues, offering no further explanation.

They landed normally – over an hour late. Various agencies, including police and the FBI were waiting. Both pilots were immediately breathalyzed. A follow-up drug screening was performed for five major substances of abuse. All tests returned negative results.

Following an NTSB investigation, the probable cause was “The flight crew’s failure to monitor the airplane’s radio and instruments and the progress of the flight after becoming distracted by conversations and activities unrelated to the operation of the flight.”

The report revealed that controllers had become dangerously complacent about NORDO events due to their frequency, often failing to follow five-minute security notification protocols.

Though the airline requested forensic examination of the laptops, this was never performed, and the cockpit voice recorder’s 30-minute limit meant no audio evidence was available from the period of silence. Media reports suggested some speculated the pilots had fallen asleep, which they strongly denied.

The Airbus A320 functioned exactly as it was supposed to. Delta, now Northwest’s parent company, announced plans to add audible ACARS alerts that would be harder to miss.

An unfortunate irony is that the pilots were deeply immersed in studying a new scheduling system that they’d ultimately never use, despite their clean records up until this point, sadly, the FAA revoked their licenses within days.

Flight 188 stands as a watershed moment in aviation safety culture – a stark reminder that even the most sophisticated automation cannot compensate for human distraction from fundamental flying duties.

📄 SOURCES
Final report: https://finalreports.net/2009/northwest-airlines-flight-188-final-report.pdf
CVR and transcripts: https://www.faa.gov/data_research/accident_incident/2009-10-23