Northwest Airlines Flight 188

NTSB narrative

PILOTS

While Northwest Airlines flight 188 (NWA188) was in cruise flight at 37,000 feet, air traffic control (ATC) directed the flight crew to change radio frequency as the airplane approached one of the sectors in Denver ATC airspace. The first officer acknowledged the frequency change and read back the correct frequency of 132.17 megahertz (MHz). However, the flight crew did not contact ATC on the new frequency. Further, there was no radio microphone keying recorded on the flight data recorder, indicating that the crew made no attempted radio transmissions on any frequency until they transmitted on 132.125 MHz, which was the frequency for Winnipeg ATC. Winnipeg ATC directed the flight crew to contact Minneapolis ATC; the flight was operating with no radio communications (NORDO) for about 1 hour 17 minutes. Because of the proximity of the Winnipeg ATC frequency (132.125 MHz) to the last frequency acknowledged by the pilots (132.17 MHz), it is likely that the first officer began to dial in the new frequency but never completed the frequency change. (To make a frequency change, the pilot had to use a rotary dial to select the desired frequency and then push a button to activate the new frequency on the radio.) Furthermore, the first officer did not attempt to contact the next ATC controller after acknowledging the frequency change. The first officer stated that the last contact with ATC was shortly before they had dinner. Thus, the first officer was likely directed to change frequencies about the time that the captain was absent from the cockpit (due to a restroom break) and the flight attendant was in the cockpit (in compliance with security procedures), which coincided with the time dinner was served. These events may have distracted the first officer from completing the frequency change or attempting to contact the next ATC controller. Additionally, the captain’s temporary absence from the cockpit would have removed the redundant monitoring of a second pilot who may have detected the incomplete frequency change or failure to check in with ATC.

Following the captain’s return to the cockpit, the pilots became distracted by a conversation regarding changes to the bidding process (the process by which the pilots request their flight schedule for the following month) resulting from the merger between NWA and Delta. The pilots allowed this conversation to monopolize their attention and, thus, lower their capacity to monitor their radio communications, notice the lack of contact, and recognize, via airplane instruments, the flight’s progress. Both pilots stated that they heard radio chatter but did not hear a radio call for NWA188. During the pilots’ conversation, the pilots opened and operated their personal laptop computers. Postincident investigation revealed that laptops could block the pilots’ primary flight and navigation displays depending on their placement, but would most likely not block the upper screen of the electronic centralized aircraft monitor, where “ACARS [aircraft communication addressing and reporting system] MSG” blinks. The computers not only restricted the pilots’ direct visual scan of all cockpit instruments but also further focused their attention on non-operational issues, contributing to a reduction in their monitoring activities, loss of situational awareness, and lack of awareness of the passage of time. The pilots missed numerous visual alerts, including ACARS messages sent by NWA dispatch (ACARS does not have an aural alert) and at least nine messages regarding their position (on the multifunction control and display unit and the primary flight displays) as they neared Minneapolis-St Paul International/Wold-Chamberlain Airport without landing data entered into the flight management computer. Airline policy in effect at the time of the incident prohibited the use of portable electronic devices on the flight deck.

Although the pilots indicated that they had one of their radios tuned to 121.5 MHz (the universal emergency frequency), they did not respond to calls from ATC on that frequency. It could not be determined why the pilots did not respond. Possible reasons include: the volume was turned down; the pilots were distracted; or the airplane was outside of coverage for the 121.5 MHz transmitters.

ATC

NWA188 entered the first two Denver ATC sectors uneventfully; as the airplane entered the next two sectors, radio contact was not established. The controllers were preparing for a shift change, and neither sector controller’s relief briefing included information that communication had not been established with the NWA188. Although the current ATC practice of using automated information transfers (electronic radar handoffs) in some sectors provides an efficient means for tracking and handing off airplanes between controllers, there are no system-wide procedures for indicating that an aircraft has been directed to switch frequencies or has communicated with ATC. Because of this lack of standardization, NWA188 passed through two Denver ATC sectors without the controllers being aware that it had not made radio contact. When the flight was finally identified as NORDO by the next sector’s controller, almost 30 minutes after NWA188’s last transmission, controllers attempted to regain contact with the airplane by calling NWA dispatch personnel and requesting that they attempt to contact NWA188. The controllers indicated during interviews that this technique is commonly used initially for air carrier airplanes instead of transmitting on the universal emergency frequency of 121.5 MHz because of the limited range of sparsely located transmitters. However, when the pilots did not respond to the ACARS messages, one of the controllers made several attempts to contact the airplane on 121.5 MHz, but the pilots did not respond.

FAA Order 7110.65, Air Traffic Control Handbook, Paragraph 10-4-4. Communications Failures states, in part, the following.

Take the following actions, as appropriate, if two-way radio communications are lost with an aircraft. ... a. In the event of lost communications with an aircraft under your control jurisdiction use all appropriate means available to reestablish communications with the aircraft. These may include, but not be limited to, emergency frequencies, NAVAIDs [navigation aids] that are equipped with voice capability, FSS [flight service station], Aeronautical Radio Incorporated (ARINC), etc. ... e. If radio communications have not been (re)established with the aircraft after five minutes, consider the aircraft's activity to be possibly suspicious and handle the flight per FAAO JO 7610.4, Chapter 7, Hijacked/Suspicious Aircraft Reporting and Procedures.

Controllers and supervisors interviewed after this incident indicated that an air carrier airplane out of radio contact is not an uncommon occurrence for air traffic controllers, occasionally occurring as often as several times during an 8-hour shift. However, these interviews indicated that the losses of radio contact are usually short in duration and re-established quickly. This likely contributed to the controllers becoming complacent and not advising ATC managers of the loss of radio contact with NWA188 in a timely manner. In this incident, during which the airplane was NORDO for more than 1 hour, failure to quickly advise managers resulted in a delay in the completion of necessary actions and notifications required by the lost-communications procedures.

The NTSB makes the following conclusions regarding this incident. • The first officer acknowledged but never completed the assigned frequency change due to interruptions, likely during the time that the captain was absent from the cockpit and the flight attendant was in the cockpit. • Not completing the frequency change, contacting ATC, and maintaining communications was contrary to Federal Aviation Regulations and airline procedures and was not noticed by the flight crew because they became distracted by a conversation, during which they referenced their personal computers, focusing the pilots’ attention on non-operational issues. • The pilots’ conversation and use of personal computers led to reduced monitoring activities, loss of situational awareness, and lack of awareness of the passage of time. • Sufficient visual cues, including ACARS messages and flight progress alerts, were presented to the flight crew, but the crew did not notice any of them. • Air traffic controllers did not follow procedures to ensure NWA188 was on the correct frequency, which delayed the identification of NWA188 as NORDO. • No national standardized procedures exist when automated information transfers are used instead of the paper flight progress strips to nonverbally document and confirm air traffic control information among controllers. • ATC management did not complete the required notifications for a NORDO airplane in a timely manner as required by Federal Aviation Administration directives.

Verbatim NTSB analysis and probable cause from the NTSB dataset