737 MAX fatal event rate is much higher than that of prior 737 models

Wreckage recovery from Ethiopian 737 MAX crash (AP photo)

The 737 MAX, the newest generation of the 737 family of airliners, has had two fatal crashes in its first two years of service, giving it rate of fatal events that is several times higher than the combined rate of the previous generations of the 737. These crashes have led to the worldwide grounding of the aircraft, and a loss of public confidence in the safety of the 737 MAX. Restoring the public’s confidence will take a combination of redesigning the aircraft to avoid the problems that led to the recent crashes, along with Boeing avoiding public actions that may undermine the public’s acceptance of Boeing’s efforts.

A very brief history of the 737

The 737 prototype and former NASA test aircraft— N515NA

The 737 first entered airline service in 1968. Since then, more than 10,000 737s have been built in numerous variations, with the latest generation being the 737 MAX, which first entered airline service in 2017.

These aircraft can be thought of as being in four different generations based on significant differences in design and technology: the early models (737–100/-200), the 737 Classic (737–300/-400/-500), the 737 Next Generation (737–600/-700/-800/-900), and the 737 MAX (737 MAX 7/8/9/10). As of March 2019, about 4,700 737s are on order and about 4,600 of those are from the 737 MAX generation.

The 737 has gotten less risky over time, until the 737 MAX

At AirSafe.com, one way that risk is measured is by dividing the number of fatal events associated with an aircraft model by the number of flights performed by that aircraft model. A fatal event, in this case, is defined by any event where a passenger dies while they are in a plane during an airliner flight (details on how AirSafe.com defines a fatal event).

Over the last several decades, the risk associated with airline travel, whether from a fatal crash or from less serious events, has gone down as the industry has learned from past problems and used that knowledge to improve technology, maintenance, training, and operational procedures. For the 737, this has been reflected in the reduction of the fatal event rate (fatal events per million flights) of the first three generations of the 737:

  • 737–100/-200: 0.86 per million flights (58.3M flights, 50 events)
  • 737 Classic (737–300/-400/-500): 0.23 per million flights (79.6M flights, 18 events)
  • 737 Next Generation (737–600/-700/-800/-900): 0.09 per million flights (100.3M flights, 9 events)

The trend in the first three generations of the 737 is clear, with each subsequent generation of the 737 having a lower rate of fatal events than the previous generation. That trend has been reversed with the 737 MAX:

  • 737 MAX (737 MAX 7/8/9/10): 3.08 per million flights (0.65M flights, 2 events)

The current rate for the 737 MAX is an order of magnitude higher than the rate for any of the previous 737 generations. To give you an idea of how high this rate is, the fleet of present and future 737 MAX aircraft would have to fly an additional 21.6 million flights, 33 times the number of flights the fleet has already flown, before it would have the same fatal event rate as the previous generation of the 737.

Estimating the 737 fatal event rate

The risk values given above were computed based on two different sources. The list of 737 fatal events was provided by AirSafe.com. The list includes 79 known events where a passenger on an airline flight was killed while traveling on a 737.

The estimates for the number of flights for each 737 generation came from a review of the Boeing publication Statistical Summary of Commercial Jet Airplane Accidents. To estimate the number of flights through the end of March 2019, the data from the last two publications were used to estimate the trend for the number of flights each generation would have completed by the end of that time period.

Because the 737 MAX had no serious events before the end of 2017, the most recent Statistical Summary, which covered the years 1959–2017, had no information that could be used to estimate the number of 737 MAX flights. Instead, the following facts and assumptions were used to estimate the total number of flights:

  • According to the FAA in a March 2019 Airworthiness Notification, there were 387 737 MAX aircraft in operation.
  • The 737 MAX entered service in late May 2017 and the worldwide fleet was grounded by the middle of March 2019.
  • It was assumed that the average 737 MAX flew an estimated five times a day or about 150 times per month.
  • It was assumed that each 737 MAX aircraft mentioned in the FAA Notification was active for half of the 22 months between May 2017 and March 2019.
  • The estimated number of flights was the product of the number of aircraft in the FAA notification, the assumed number of active months for each aircraft, and the assumed average number of flights per month.
  • The estimated number of 737 MAX flights was 638,550, which was then rounded up to 650,000.

Risk vs. Safety and the future of the 737 MAX

As mentioned earlier, the 737 MAX has a substantially higher rate of fatal events per million flights than any of the previous generations of the 737. The risk associated with this model, which in this case is determined by dividing the number of fatal events by the number of total flights, is something that Boeing and their customers can reduce over time if the 737 MAX fleet keeps flying and does not experience any new fatal events. The public’s perception of the risk is something altogether different. Changing that from a generally negative perception to one that is at least neutral will take both time and effort.

The most important factor in reducing the risk is the efforts that will be made by the aviation industry, primarily Boeing and the FAA. These two organizations, which were responsible for the design and certification of the 737 MAX, will also be responsible for resolving the problems that led to the two crashes. Over time, if the problems that led to the two crashes not reoccur, then this will demonstrate to the world that the risk has been properly managed.

Changing the public’s perception of the safety of the aircraft, which will be based in part on the public’s perception of the acceptability of the actions taken to put the aircraft back into service, will likely be mostly in the hands of Boeing. Given the pervasive nature of the news media coverage and the social media attention that these crashes have generated over the last several months, it seems that the public’s interest in the 737 MAX will still be quite high months or even years after the aircraft returns to service.

It isn’t clear what Boeing should do to restore the public’s confidence in the 737 MAX, but in my opinion, there are two things that Boeing should do:

  • Avoid having Boeing leadership and its communications department repeatedly state that the 737 MAX is safe rather than directly addressing the concerns of the airlines and the traveling public.
  • Avoid even the appearance of an inappropriate relationship between Boeing leadership and the FAA, the NTSB, or any other part of the US government that is involved in either the investigation of the 737 MAX accidents or in approving the aircraft’s return to service.

Resources

AirSafe.com description of the differences between risk and safety

Chapter 6 from Understanding Aviation Safety Data

Common definitions used by AirSafe.com, the FAA, and the airline industry

Definitions of fatal event as used by AirSafe.com

FAA Continued Airworthiness Notification concerning the 737 MAX dated March 11, 2019

Article on the unusually high public interest in the crash of an Ethiopian Airlines 737 MAX

Former USAF and Boeing engineer and creator of aviation safety and security site AirSafe.com.

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