While the first generation 737-300 was a huge success, building on the already popular range of Boeing domestic aircraft, the company was still interested in expanding its portfolio with longer range, increased capacity, reduced noise and improved fuel efficiency. Enter the 737-400, which is considered by many to be the cream of the 737 Classic crop.
The 737-400 however was not considered the be an expansion of the preceding 737-300 from the outset, as initial considerations for the -300 were that it wouldn’t be that great a success, seeing as it had had a somewhat turbulent development with regard to its engines. However, in spite of this, the -300 was a colossal hit, and thus Boeing were willing to strike while the iron was hot. Development on a larger version of the 737-300 began in 1985, its primary purpose being to bridge the gap between the 737-300 and the 757-200, as well as make it competitive with the upcoming Airbus A320 and the McDonnell Douglas MD-80 Series.
The primary changes to the design of the 737-400 are an extension of the fuselage by 10ft, so as to increase capacity for up to 188 passengers. However, since the 737 is a very low aircraft, its longer body means there were chances the tail would strike during rotation or landing, a problem found early on in the development of the 757. As such, Boeing also fitted the aircraft with a bulbous tail bumper to protect the hull from impacts. To accommodate the extra weight, the -400 was also given a strengthened wing spar, and flap limiting speeds were increased over the -300. Other changes included two extra overwing exits for the increased capacity, and a boost to the air-conditioning system to give the necessary increase in air flow rate.
Power came from the same CFM International CFM56-3C-1 engines that powered the -300, but the aircraft’s increased weight meant it’s range was less than that of the -300, being only 2,060nmi as opposed to its older brother’s 2,255nmi. In terms of speed and cruising altitude however it was identical to both the earlier 737-300 and the later 737-500.
The prototype left the Renton factory on January 26th, 1988, and performed its maiden flight on the 19th February. After extensive testing, the aircraft was given FAA Certification in the summer of that year, being delivered to launch customer Piedmont Airlines on September 15th, the first of 25 aircraft ordered by that airline. While the 737-400 was popular and respected among customers, it didn’t really have the same impact as the earlier -300. The -300 was seen as a more compact aircraft, allowing it to be more flexible on routes to airports with limited space. For work on busier routes requiring greater capacity, airlines usually opted for the larger Boeing 757-200, which meant the 737-400 fell into something of a hole. It did sell quite well though, with 486 aircraft leaving the factory and working with some of the world’s premier airlines, including British Airways, British Midland, Alaska Airlines, US Airways, Lufthansa, Scandinavian Airlines and many more.
Officially, the 737-400 was not available in a freight variant, although a 737-400F was proposed early on but never reached the development stage. Instead, many 737-400’s have been converted by airlines for use as either Combi aircraft or as dedicated freight aircraft. Alaska Airlines pioneered these conversions in the 1990’s for use on freight flights to the more remote regions of the arctic nation, the aircraft being able to carry up to 10 cargo pallets. The 737-400 was also not available with Winglets, but, much like the 737-300, this has now become an option, with Winglets provided by Aviation Partners.
However, though the -400 was popular, it wasn’t without its faults, chief among which were those very important little pieces of equipment, the engines. In total, the 737-400 has been involved in 10 incidents resulting in 301 fatalities.
Perhaps the most infamous incident involving the 737-400 was on the night of January 8th, 1989, when British Midland Flight 92 crashed short of the runway at East Midlands Airport while performing an emergency landing. The aircraft had earlier suffered a fan blade fracture on the port-side engine, with the crew, being unfamiliar with the aircraft, accidentally shutting down the starboard engine which was still functioning normally. The result was the aircraft losing power, stalling and then striking the ground in a field near Kegworth before plunging into a bank on the opposite side of the M1 Motorway. The crash resulted in the deaths of 47 of the 188 passengers and 8 crew. It was later found that the CFM56 was prone to fan blade fractures, and due to flight testing of new engine variants not being mandatory, the problems were not highlighted earlier in the development stage. It was found that the fan was being subjected to high-cycle fatigue stresses worse than expected and also more severe than tested for certification; these higher stresses caused the blade to fracture. Less than a month after grounding, the fleet was allowed to resume operations once the fan blades and fan disc were replaced and the electronic engine controls were modified to reduce maximum engine thrust to 22,000lbf from 23,500lbf . The redesigned fan blades were installed on all CFM56-3C1 and CFM56-3B2 engines, including over 1,800 engines that had already been delivered to customers.
Aside from the Kegworth Disaster, the 737-400 has been involved in other incidents and accidents, though surprisingly with quite high survivability rates considering the impacts. While the first loss of a 737-400 was at Kegworth, the first fatal crash not due to engine issues was Turkish Airlines Flight 278 on December 29th, 1994, which crashed while attempting to land at Van Ferit Melen Airport in Van, in eastern Turkey, resulting in the deaths of 57 of the 69 on board. At the time this was the worst accident to involve the 737-400.
This would be later surpassed on January 1st, 2007, when Adam Air Flight 574 disappeared off the coast of Sulawesi in Indonesia, killing all 102 aboard. The final report, released in 2008, concluded that the pilots lost control of the aircraft after they became preoccupied with troubleshooting the inertial navigation system and inadvertently disconnected the autopilot. The crash however opened a can of worms when it came to the safety of Indonesian aviation, combined with a later incident involving Adam Air Flight 172, which involved a 737-300 landing so hard the aircraft broke its back, but fortunately with no injuries. The mixture of poor maintenance and improper crew training resulted in a large-scale transport safety reform in Indonesia, as well as the United States downgrading its safety rating of Indonesian aviation, and of the entire Indonesian fleet being added to the list of air carriers banned in the EU. Adam Air was subsequently banned from flying by the Indonesian government, and later declared bankruptcy.
The latest crash of a 737-400 was a cargo converted aircraft on August 5th, 2016, which overshot the runway at Milan Bergamo’s Orio al Serio Airport while landing. Upon leaving the runway the aircraft smashed through the perimeter fence and a car park before coming to rest on a main road. Despite losing both engines, the pilots managed to escape with minor injuries.
Today, around 259 737-400’s are known to still be in active service with a wide variety of airlines, and while most have lost their prestigious positions with larger carriers such as British Airways and US Airways, they continue to form the backbone of a wide range of low-cost and smaller carriers across the globe. The saddest part of the 737-400’s story is the fact that it fell in a gap in the market between the smaller 737-300 and the 757. The problem with the 737-400 is it didn’t add anything apart from capacity, being just as fast and flying as high as the -300, but being unable to fly the same range.
At the end of the day, in spite of the advantages it had with a larger hull, the -400 was seen as a bit of a damp squib; a useful squib, but a squib nonetheless.