[2.0] 737 NG, 737 MAX, & Military 737s

v1.2.0 / chapter 2 of 2 / 01 nov 17 / greg goebel

* Boeing was not satisfied with the second generation of the 737, moving on to the "737 New Generation" in the 1990s, with improved CFM56 engines and a redesigned wing. The company is now moving forward on the fourth generation, the "737 MAX" series.

Although the 737 was purchased mostly by commercial airlines, it was also obtained by military forces -- not just as transports but for other roles, such as the US Air Force "T-43A" navigation trainer, based on the 737-200. The US Navy is now obtaining the "P-8 Poseidon" maritime patrol aircraft, based on the 737. This chapter discusses the third and fourth generation 737s, as well as specific military variants.

Boeing 737-800

[2.1] 3RD GENERATION 737-600, -700, -800, & -900 / BOEING BBJ
[2.3] MILITARY 737S (1): T-43A / SURVEILLER
[2.5] MILITARY 737S (3): P-8 POSEIDON

[2.1] 3RD GENERATION 737-600, -700, -800, & -900 / BOEING BBJ

* Even as the second-generation Boeing 737s were becoming established in service, Boeing officials were considering a third generation to keep up with aggressive competition, as well as replace the Boeing 727 -- the Boeing 757 had been supposed to do that job, but it ended up appealing to a slightly different market. In 1993, the company kicked off the "Next Generation 737-X" program. As the program emerged, the 737 NG machines shared several significant improvements:

Boeing 737-700 with winglets

Four basic 737 NG variants were built. The first to fly was the "737-700", originally "737-300X", which was to replace the 737-300 and could be fitted with a maximum of 149 seats. Initial flight was on 9 February 1997, aircrew being Mike Hewett and Ken Higgins. Certification and initial deliveries took place later in that year. It would lead to specialized subvariants, see below.

   BOEING 737-700:
   _____________________   _________________   _______________________
   spec                    metric              english
   _____________________   _________________   _______________________

   wingspan                34.31 meters        112 feet 7 inches
   wingspan (winglets)     35.79 meters        117 feet 5 inches
   wing area               125 sq_meters       1,345.5 sq_feet   
   length                  33.63 meters        110 feet 4 inches
   height                  12.57 meters        41 feet 3 inches

   empty weight            38,145 kilograms    84,100 pounds
   MTO weight              70,080 kilograms    154,500 pounds

   max speed at altitude   1,015 KPH           630 MPH / 550 KT
   cruise altitude         12,500 meters       41,000 feet
   range, full load        2,850 kilometers    1,770 MI / 1,540 NMI
   _____________________   _________________   _______________________

The second was the "737-800", originally "737-400X Stretch". It had a length of 39.47 meters (129 feet 6 inches) and a maximum capacity of 189 seats. Initial flight was on 31 July 1997, with introduction to service in the spring of 1998.

The "737-600", originally "737-500X", was the third, intended to replace the 737-500. It had a length of 31.24 meters (102 feet 6 inches) and a maximum capacity of 132 seats. Initial flight was on 22 February 1998, with introduction to service before the end of the year. Incidentally, Boeing never offered winglets as a purchase option on the 737-600; it is unclear if any aftermarket vendors offered them.

Boeing 737-600, 737-700, 737-800, 737-900

The fourth, the "737-900", didn't fly until 3 August 2000, with introduction to service in the spring of 2001. It was the biggest 737 built to that time, with a length of 42.11 meters (138 feet 1 inch) and a maximum capacity of 215 seats -- well more than the 189 seats of the 727-200, the irony being that the 737, designed as a "baby brother" to the 727, ended up a bigger aircraft.

Actually, at the outset the 737-900 could only carry a maximum of 189 passengers due to regulatory issues. It led to "737-900ER", introduced in 2007, which could handle a full 215 passengers, thanks to the addition of two more emergency exits. The 737-900ER could also be obtained retaining the maximum of 189 seats and without the additional emergency exits, but with additional tankage for improved range. Of course, the airframe was reinforced to handle higher MTO weight.

* From 1996, Boeing began work on an increased growth weight "737-700IGW" which was actually a hybrid, effectively a 737-700 with the stronger wing and landing gear of the 737-800, permitting additional fuel tankage and special equipment fits. The primary initial motivation of this exercise was the "Boeing Business Jet (BBJ) version of the 737, intended for the executive-VIP market. It proved surprisingly popular.

The 737-700IGW airframe also led to the "737-700C" and "737-700QC" combi freighter aircraft; and then the extended-range "737-700ER", featuring additional fuel tankage. It had a range of 10,200 kilometers (6,335 miles / 5,510 NMI), about twice the range of the baseline 737-700. It could be configured for 48 seats in a premium configuration, or up to 126 seats in a traditional two-class configuration. Deliveries began in 2007. Boeing also introduced a "BBJ2", based on the 737-800, and the "BBJ3", based on the 737-900ER, with a cargo hatch as an option.



* By the 21st century, the 737 was one of the world's most successful airliners. In 2005, one in four of the world's large airliners were 737s; 6,100 had been ordered in total, with more than 4,100 in service with over 400 airlines; and over a thousand were in the air at any one time. Of all the commercial jetliners sold by Boeing and Douglas, a good third of them were 737s. Ironically, the 737's old rival, the DC-9, had evolved through the acquisition of McDonnell Douglas by Boeing into the "Boeing 717", and then had ceased production.

Late in the first decade of the 21st century, Boeing was focused on development of the "787" advanced technology twinjet, leveraging off 787 technology to also update the 747 jumbo jet to the "747-8". The company had considered development of an all-new replacement for the 737 -- but given customer satisfaction with the 737 and a short-term demand for something better, Boeing decided to give the 737 a 787-flavored facelift as well.

Rival Airbus had been similarly considering a new-design replacement for its competing A320, but decided to introduce an updated version, the "A320 New Engine Option (NEO)", instead, with delivery from 2015. Boeing couldn't afford to leave the field to Airbus over the longer interval it would have taken to develop a new machine that could only be an incremental improvement over an updated one -- and so decided to have the next-generation "737 MAX" flying by 2016, and in the hands of airlines by 2017.

Boeing 737 MAX

At the outset, Boeing planned three models of the fourth-generation 737 MAX: the "737 MAX-7", "737 MAX-8", and "737 MAX-9" -- updates of the 737-700, 737-800, and 737-900 respectively, all of similar configuration, but with different capacities. They were all to be offered in Boeing Business Jet configurations. The company claimed the new machines would be 10% to 12% more fuel-efficient than the previous generation of 737s. Of course, it was fuel efficiency that was pushing the upgrade; jetliners burn a lot of fuel, and as expensive as it is to buy a new jetliner, it's damned expensive to fly an old one that guzzles fuel.

The core of the upgrade was a new engine, the CFM International "LEAP-1B". The LEAP-1B has a diameter of 175 centimeters (69 inches) as opposed to the 157 centimeters (62 inches) of the CFM56. That required stretching the 737's nose gear by 20 centimeters (6 to 8 inches) to clear taxiway lighting; the configuration of electronic gear in the nose had to be rearranged to accommodate the longer nose gear. The engines have the distinctive serrated cowlings introduced on the 787 to reduce noise.

Airframe changes to the 737 MAX series were to be minimal -- no major changes of airframe elements from metal to composite materials, no new wing, though an extended tailcone and twin-fin ("dual feather") winglets were fitted. Since third-generation 737s already had a glass cockpit, there was no need to do much more there than update to current technology. Boeing also decided to leverage off the "Boeing Sky" interior developed for the 787, featuring:

Boeing Sky interior for 737

The Boeing Sky interior is also now available as an option on 737 NG machines and has proven popular. It cannot be retrofitted to earlier machines, but third parties do offer retrofits with some of the functionality. The recess around the windows in the 737 MAX, incidentally, is purely cosmetic, something of an optical illusion that implies no change to the windows themselves, with the 737 MAX retaining the same old window scheme -- they still have sliding blinds, not the fancy electrochromic dimmers found on the 787.

In the design of the 737 MAX, Boeing was very leery of "feature creep", the unpleasant tendency of "modest updates" to become comprehensive redesigns. However, it's hard to resist; in 2016, in response to customer feedback, Boeing announced that it had stretched the 737 MAX-7 relative to the 737-700 to add two more rows of seats, with plugs fore and aft of the wings to increase length to 35.56 meters (116 feet 8 inches), providing 172 seats in an all-economy configuration and 138 in a two-class configuration.

With the greater seat capacity, two emergency exits were fitted over each wing instead of one; given the greater takeoff weight, the 737 MAX-7 was given the MAX-8's wings and landing gear. Nonetheless, there was otherwise no basic change in configuration. Indeed, the 737 MAX noticeably retains a feature distinctive to all 737s from the beginning of its service: there's still no main gear doors.

Initial flight of the first 737 MAX prototype, a 737 MAX-8, was on 30 January 2016, with three more prototypes following later. Initial deliveries to customers were in the spring of 2017, Boeing having moved up the schedule as much as possible to deal with A320NEO competition. Boeing is planning to introduce a high-density "sardine can" version of the 737 MAX-8, named the "737 MAX 200", in 2019, this model featuring another exit door to cope with the greater passenger load.

Initial flight of the first of two 737 MAX-9 prototypes was on 13 April 2017. It is expected to be certificated and delivered in early 2018. Initial flight of a 737 MAX-7 will be in 2018, with introduction to service in 2019.

At the 2017 Paris Air Show, Boeing introduced another MAX variant, the 737 MAX-10" -- originally "737-10X" -- to counter the stretched Airbus A321NEO. The 737 MAX-10 is stretched 1.67 meters (66 inches) relative to the 737 MAX-9, for a length of 43.8 meters (143 feet 8 inches). It will have seating for 230 in a single-class configuration, and 189 in a dual-class configuration. Avionics will be updated, more electric systems added, and the design will feature next-generation composite wings. Powerplants will be uprated LEAP-1B turbofans. The 737 MAX-10 is expected to enter service in 2020.

Boeing envisions a total market of 23,000 mid-sized airliners to 2030. The company was sharply criticized for waffling on a new-design replacement for the 737 and then deciding to simply go for an update, lagging Airbus by several years -- but better late than never, and orders for the 737 MAX have been doing well. It isn't hard to believe that the 737 may still be flying passenger service in 2030 or even beyond, a testimony to the staying power of a good design.


[2.3] MILITARY 737S (1): T-43A / SURVEILLER

* Boeing's successful 737-200 attracted military attention from early on, with the US Air Force (USAF) obtaining 19 "T-43A" navigation trainers in 1973 and 1974. They had workstations and facilities for twelve student navigators, four advanced students, and three instructors, with the floor reinforced to handle the training gear. There were five periscopic portholes in the roof for sextant sightings; the T-43As also had an auxiliary fuel tank for extended range. They looked pretty much like stock 737-200s externally, except for a reduced number of windows and more antennas cluttering up their lines. These aircraft were known as "Gators", it seems not because of anything reptilian in their appearance, but because they trained "(navi)gators". Nobody ever accused the military of having an overly subtle sense of humor.

Boeing T-43A

Six were reconfigured to a staff / command transport configuration as the "CT-43A", one being lost in a high-profile crash in Croatia on 3 April 1996 that killed all on board, including US Commerce Secretary Ron Brown. The aircraft flew into a mountain while coming in to land at Dubrovnik airport; investigation concluded it was due to flight crew error, the crew not being familiar with the relatively antique approach procedures at Dubrovnik, with foul weather contributing to the accident.

The CT-43As were retired in 2001, being replaced by the C-37, a variant of the Gulfstream V business jet. The last T-43As were retired in 2010, being replaced by ground simulators backed up by flight training in T-1A Jayhawks -- military conversions of the Raytheon Hawker 400 business jet. However, one does fly as the "NT-43A", a T-43A refitted with radomes fore and aft, operating as a test system to evaluate "stealthy" aircraft in flight. It also features sensors, presumably infrared imagers, on top of the nose and tail radomes. The aircraft went into service in its new form in 2001. Not much is known about the NT-43A; there's probably nothing all that secret about the NT-43A's gear, but it works on secret programs, and so ends up being necessarily covered by the same cloak of secrecy.

NT-43A & JSF trials platform

Other 737-200s have been used for electronics test and trials platforms, one being used by the Elta division of Israel Aircraft Industries for various equipment fits, and another given a "nose job" by Boeing to test gear for the USAF's F-35 Joint Strike Fighter (JSF).

* The only other 737-200 machines built as distinctly military platforms were three "727-2N3 Surveiller" maritime patrol aircraft sold to the Indonesian Air Force in the early 1980s. They looked exactly like stock 737-200s except for twin strakelike 5-meter (16 foot 5 inch) antenna fairings for the Motorola "AN/APS-135(V) Side-Looking Airborne Multi-Mission Radar (SLAMMR) on top of the rear fuselage, flanking the tailfin.

The SLAMMR system was capable of picking up small ocean craft from a range of up to 160 kilometers (100 miles) while the aircraft was flying at cruise altitude. It does not appear that the Surveillers had many other optimizations for the maritime patrol role, though they may have been also fitted with signals intelligence (SIGINT) gear; in fact, they could still be used as personnel transports with 92 seats. They remain in service at last notice; they were given equipment updates in the early 1990s and may have received further updates later.

Incidentally, the Indonesian Air Force also obtained several 737-200 (and 737-400) machines as VIP / personnel transports. A fair number of 737-200s were flown as VIP / personnel transports in service with military forces in nations around the world, somewhat surprising users including India and Iran.



* The US military went on to obtain CFM-powered 737s. Both the US Navy (USN) and the USAF were interested in the 737-700 BBJ, with both services obtaining the type as the "C-40 Clipper". The Navy initially ordered a total of twelve "C-40A" machines, which were in a "combi" configuration, featuring the large cargo door on the left forward side of the fuselage, plus an interior that could be reconfigured to haul 121 passengers, or eight cargo pallets with a total load weight of 18,140 kilograms (40,000 pounds), or a mix -- 70 passengers and three cargo pallets, for example.

Early C-40A deliveries did not have winglets; later deliveries did, with winglets retrofitted to the earlier aircraft. Deliveries were from 2001 into 2011; they were obtained for the Naval Reserve and were intended to replace C-9B (DC-9) transports. The Navy ordered two additional C-40A transports in 2012, for a total of 14, and would like to get more; the C-9Bs were retired in 2014.

US Navy C-40A

The Air Force's "C-40B", in contrast, is a VIP transport, lacking the cargo door, and intended for transport of high US military and government officials. It typically carries 11 crew and 26 passengers in relatively luxurious accommodations, including a crew rest area, VIP compartment with sleep accommodations, two galleys, and a "business class" section with worktables. It also has a high-bandwidth secure global communications system and auxiliary fuel tanks. A total of four have been obtained. Some of the early deliveries didn't have winglets, but they were upgraded with them later.

US Air Force C-40B

The Air Force also obtained "C-40C" combi machines, essentially the same as Navy C-40A aircraft, with seven obtained so far. While it can be hard to spot the front cargo door in pictures of the C-40C, it is easy to tell it from the C-40B, since the C-40B has a clutter of antennas on its fuselage to support its advanced communications systems, as well a distinctive antenna fairing on top of the tailfin. All C-40Cs were delivered with winglets.

* CFM-powered 737s have also been acquired by the armed forces of several nations as VIP / personnel transports, sometimes with additional communications and self-defense kit. There's a tale that the USAF obtained a 737-800-based VIP transport, but it is difficult to find any details or a US military designation for this machine; the Royal Saudi Air Force does have a 737-800-based VIP transport, and the USAF may have been acting as an "agent" for the Saudis in its purchase.

There have been some special-mission conversions of CFM-powered 737s. In 2010, a 737-300 was observed at a Chinese air show with a set of antenna fairings top and bottom. It was said by authorities to be one of two machines obtained second-hand from Garuda Indonesia and converted into missile tracking platforms. Details were not forthcoming. There are tales of other Chinese special-mission conversions of the 737-300, such as an "airborne command post", but they may be just garbled reads of reports about the missile tracking aircraft.

Chinese 737-300 missile tracking platform

* One of the most sophisticated militarized 737 variants is the "Wedgetail" early warning aircraft, more formally known as the "737 Airborne Early Warning & Control (AEW&C)", the informal name being in honor of the Australian wedgetail eagle. The aircraft consists of a 737-700 BBJ airframe fitted with a Northrop Grumman "Multirole Electronically Scanned Array (MESA)" radar system, which is built around an "active array" of transmitter-receiver (T/R) elements, mounted in a surfboard-like radome on a pedestal mounted on the aircraft's spine. The T/R elements are each like a radar unto itself; they can be individually programmed to allow the array to perform different functions -- not just radar, but also communications, electronic intelligence, and electronic countermeasures. Multiple functions can be performed at the same time by allocating different parts of the array to different functions.

Initial test flights were in 2005. Australia bought six, with deliveries from 2006 to 2008. Turkey has four on order. South Korea ordered four in 2006, with deliveries in 2011 and 2012, with Qatar ordering three in 2014. It is a very sophisticated system and needs to be discussed in detail elsewhere, in a document on Boeing-built AEW platforms.


[2.5] MILITARY 737S (3): P-8 POSEIDON

* The US Navy is also acquiring a highly sophisticated maritime patrol aircraft based on the 737, the "P-8 Poseidon". During much of the Cold War, the mainstay of ocean defense of the West was the Lockheed P-3C Orion, a four-turboprop maritime patrol aircraft. The Orion was built in large quantities and not only used by the US Navy (USN) but by many American allies. By the end of the 20th century, however, the Orion was beginning to show its age, and the USN began to cast about for a replacement. After a frustrating search, in 2004 the USN finally awarded a contract to Boeing for the "P-8A Poseidon Multimission Maritime Aircraft (MMA)", with plans to obtain up to 117 machines.

The P-8A is a militarized 737, based on the fuselage of the 737-800 fitted to the wing and landing gear of the 737-900ER. It is clearly a weapon system, with a weapons bay in the fuselage for homing torpedoes and depth charges; four underwing pylons for Harpoon antiship missiles or other stores; and a rotary dispenser for up to 126 sonar buoys ("sonobuoys"), or stores designed to the sonobuoy form factor.

Boeing P-8A Poseidon

The primary combat avionics sensor is the Raytheon AN/APY-10 radar system, installed in the nose. The AN/APY-10 is a multimode search radar, with a "synthetic aperture radar (SAR)" capability for detection, imaging, classification, and identification of large or stationary vessels or for ground surveillance. The radar also features an "inverse SAR (ISAR)" capability to characterize small or fast moving vessels; has wide-area and high-resolution "spotlight" modes; and possesses passive signals intelligence (SIGINT) capabilities, allowing it to detect, characterize, and locate a wide range of radio or radar "emitters". In addition, the P-8A carries a Wescam electro-optical / infrared camera turret for visual inspection of targets.

Other avionics include a GPS / inertial navigation system unit; a suite of communications gear for voice and datalink using direct transmission or satcom relay; and a self-defense suite, including a radar warning system, a heat-seeking missile dazzler, and towed decoys to divert radar-guided missiles. Crew includes pilot and copilot, plus five or six mission crew.

* Two static-test prototypes and three flight prototypes were ordered at the outset, the first making its initial flight on 25 April 2009. Three more flight prototypes were ordered later. Introduction to operational service was declared on 1 July 2013, with operational flights taking place before the end of the year.

Orion crews are glad to get their hands on the Poseidon, not least because it's a nicer ride: the Orion tends to fly rough, while the P-8A has the smooth quiet flight of a jetliner, an important consideration when flying long and exhausting patrols. Cockpit crew like the glass cockpit, though they say it doesn't really make life much easier for them, because the Poseidon has more functional capability than the Orion -- like running faster to stay in the same place. Fuel burn of the P-8A is poorer than that of the P-3C down low, but the fact that the P-8A can get to station faster compensates for that drawback; the P-8A has a higher ceiling, giving a bigger patrol "footprint". Landing roll is somewhat longer, the P-8A's thrust reversers not being as effective as the P-3C's reversible props.

The "tactical coordinators (TACCOs)" also like the aircraft's "Mission Computing & Display System (MCDS)", which allows them to display overlaid data from the aircraft's sensors and remote datalinked systems on the twin 61-centimeter (24 inch) displays of each TACCO workstation. The MCDS provides an annotated view of the battlespace, integrating inputs from up to 64 sonobuoys along with radar and video. The system includes a reference library to provide useful information on identified targets, as well as a stores menu to allow the TACCOs to select stores for deployment or launch. The TACCOs also will be able to communicate with other platforms over secure comlinks, including access to and control over unmanned aerial vehicles.

Boeing P-8A Poseidon

The P-8A is designed with upgrades in mind. Software will be updated every two to three years; to support new hardware, the aircraft provides 50% more power and cooling capability than needed at the outset and can handle almost 11 tonnes (12 tons) more payload. A new radar is in the works -- see below -- as well as high-tech stores:

HAAWC is on track for deployment, though the expendable drone program still experimental. The rationale for these two stores is that they would reduce the transit time and fuel burn required for the P-8 to descend and then return to cruise altitude. This is not only more economical in terms of fuel, it also means less stress on the airframe, and a longer service life.

In 2009, the P-8A was also selected by the Indian Navy as the "P-8I", with India to acquire eight machines, the first being delivered in late 2012, the last in 2015. Four more were ordered in 2016. They are much the same as the US Navy P-8As, though with some Indian avionics factored in; they were also delivered without secure communications and electronic warfare systems, India not having signed the proper security paperwork at the time. India is also obtaining the Harpoon anti-ship missile and US torpedoes to arm their P-8Is. India was a high-profile sale for Boeing, since traditionally India has bought little military hardware from the USA.

Australia confirmed an order for eight P-8As in 2014, towards a total order of 15, with initial delivery in 2016. The UK ordered nine in late 2015, with initial deliveries in 2020. Norway ordered five in 2017, to be delivered in 2021:2022. Boeing expects more foreign sales of the type, particularly from nations that have been traditional users of the P-3 Orion.

* The US Navy received its 50th P-8A at the beginning of 2017, with the last of the order to be delivered in 2019. An inflight refueling capability, using boom refueling, will be introduced into service late in 2017 or in 2018.

Work is underway for an "Increment 2" upgrade to the P-8A, planned to be introduced before the end of this decade. The heart of the upgrade is the Raytheon "Advanced Airborne Sensor (AAS)", an active array radar system derived from the AN/APS-149 Littoral Surveillance Radar (LSRS) carried on some Navy P-3C Orions, and featuring a range of sophisticated surface surveillance modes. The AAS will be fitted in a long ventral canoe; it turns out that the selection of the 737-800 as the basis for the Poseidon was driven by the length of the canoe. Increment 2 is being followed by an "Increment 3", which appears to be focused on an enhanced, reconfigurable battle management system, with introduction in 2020.

Boeing has been making broad comments about a "Medium-Sized Maritime Surveillance Aircraft" based on the P-8A, presumably based on a smaller 737 variant with a subset of the P-8A's capability. The USN has also been searching for a replacement for their airborne electronic intelligence (ELINT) platform, the EP-3E Aries, much like the P-3C except for payload. This requirement has proven even more difficult to deal with, and in fact at the present time no competition has been initiated to obtain the Aries replacement. Boeing has come up with notional designs for the "EP-X", based like the P-8A on the Boeing 737. Boeing envisions the EP-X as carrying 14 operator stations and an advanced active-array radar system.

Ideas are being floated for modular payloads that could be swapped out to tailor the EP-X to different missions. For the time being, the EP-X is going nowhere -- but Boeing is also pushing the 737 as a replacement for the Air Force's 707-based E-8 "Joint Surveillance Target Attack Radar System (Joint STARS)" battlefield surveillance platform. The 737-700 variant would be used as the basis for the Joint STARS replacement. A downselect for for the Joint STARS replacement will take place in late 2017.

Incidentally, when news of China's conversions of 737s to missile tracking platforms broke, there was some fussing in the defense blogosphere about the negligence of the USA in allowing China to use an American jetliner for military purposes, and for the fact that these Chinese special-mission platforms were in service so much more quickly than the P-8A.

It was a tempest in a teacup. The US government has little control over old jetliners sold on the international market, and comparing the Chinese conversions to the P-8A was like comparing a car given an updated sound system to one converted into an armored battle wagon. The conversions were no more challenging than that applied to the NT-43A; the Chinese could have used a wide range of older aircraft, it was just a question of installing special-purpose kit in some airframe that could truck it around. The P-8A, in contrast, is an optimized weapon system integrating a suite of sensors, armament, communications gear, and self-defense systems. Whatever problems have afflicted the P-8A program, it's in a different league.



* The following table gives baseline 737 variants, including maximum passenger capacity and length in meters for comparison:

                  engine   seats   length  

   1st-generation 737s:

   737-100          JTD8     103    28.65
   737-200          JTD8     130    30.5

   737 Classic:

   737-300       CFM56-3     149    33.4
   737-400       CFM56-3     168    36.45
   737-500       CFM56-3     138    31

   737 NG:
   737-600       CFM56-7     132    31.24
   737-700       CFM56-7     149    33.65
   737-800       CFM56-7     189    39.47
   737-900       CFM56-7     215    42.11

   737 MAX:

   737 MAX-7     LEAP-1B     172    35.56
   737 MAX-8     LEAP-1B     189    39.47
   737 MAX-9     LEAP-1B     215    42.11
   737 MAX-10    LEAP-1B     230    43.8  

By the end of 2015, total deliveries of 737s were approach the 9,000 mark, with over 3,000 on order.

* Investigating the 737 led me to finally dig into something that had been puzzling me for a long time: the difference between a leading-edge slat and a leading-edge flap. I was half-tempted to think they were just different names for the same thing, but since the 737 had both leading-edge slats and leading-edge flaps, that couldn't be the case.

It took some poking around since the topic is not well documented -- even when it's discussed, it's not always discussed very clearly -- but the two technologies are distinct. Some short-takeoff aircraft have "slots", which are fixed narrow airfoils mounted directly ahead of the wing, their function being to direct airflow over the wing at low speeds. Slots are draggy, however, and not useful for high-performance aircraft, and so machines like the 737 have "slats", which are essentially retractable slots. Sometimes slots are called "fixed slats".

Leading-edge flaps, in contrast, are essentially like trailing-edge flaps, surfaces which are turned downwards to increase wing curvature for improved low-speed handling. There are different approaches to leading-edge flaps, one being a "drooping" leading edge flap -- just like it says, it's hinged at the rear and drops down to increase lift. The "Kruger flap" on the 737 is a little less intuitive, being something like a door hinged on the leading edge of the wing. For takeoffs and landings, a Kruger flap essentially "flips over", going from being closed under the leading edge of the wing to open forward into the airstream, the rear edge of the "door" then becoming the front edge. It looks a little awkward, but it works just fine. No doubt there are other variations on leading-edge flight surfaces, but trying to figure out that much was enough of a pain.

Boeing 737-800

* As concerns copyrights and permissions for this document, all illustrations and images credited to me are public domain. I reserve all rights to my writings. However, if anyone does want to make use of my writings, just contact me, and we can chat about it. I'm lenient in giving permissions, usually on the basis of being properly credited.

* Sources include:

Of course, various volumes of JANE'S ALL THE WORLD'S AIRCRAFT were consulted for details. Wikipedia has some fair to good articles on the 737, and the "737 Technical Site" had some interesting materials as well. YouTube also turned out to be a useful resources, with videos showing how slats and Kruger flaps actually worked. Pictures are worth a thousand words; videos are worth a hundred pictures.

* Revision history:

   v1.0.0 / 01 feb 12 
   v1.0.1 / 01 jan 14 / Aussie P-8A buy, cleanups.
   v1.1.0 / 01 dec 15 / 9,000 737s, Qatar Wedgetail order.
   v1.2.0 / 01 nov 17 / More on 737 MAX, P-8, etc.