In the 1960’s Boeing designers were years behind the McDonnell Douglas DC9 and BAC 111 and knew they had a problem as these two twin engine aircraft would beat them to market gaining a considerable sales advantage.
It appears that decisions taken to try to get the 737 to market as quickly as possible back in the late 1960’s, together with other design choices, both then and more recently, have come back to bite Boeing in the bum. Both the DC9 and the 111 were rear engined, arguably a design that made the aircraft a bit more complicated as the high Tee tail makes for a more unstable aircraft. Boeing had built the B52 bomber and the 707 airliner, aircraft with engines slung under the wings, so that was the path they went down for their new 737 rather than follow the three rear engined 727. Boeing next decided to use an upper fuselage with the same lobe as the 707 and 727 so that they could carry the same size upper deck cargo pallets across all three models in any future cargo version. It also allowed 6 abreast seating, rather than the oppositions 5 abreast arrangement, allowing for a shorter aircraft for the same number of passengers. The lower fuselage was pretty standard but the overall effect was to make the plane short and dumpy, hence the ‘Fat Albert’ nickname.
To sell against the DC9 and the 111 Boeing needed advantages, the 6 abreast seating was one, but it was not enough. The designers came up with the idea that if they kept the ground clearance low they could install deployable “airstairs” in the aircraft which could be operated at the flick of a switch and the self loading cargo, otherwise known as passengers, could be moved without waiting for portable stairs to arrive. If you could cut down turnaround time and get the plane back in the air quickly the airline were happy, as a plane only makes money when it is in the air. But how to get the 737 close to the ground with under wing engines? The Boeing answer was to do away with engine pylons and to mount the engines directly under the wings. This gave the 737-100 and -200 their distinctive cigar tube engine arrangement.
In 1984 Boeing introduced what became known as the “Classic” range of 737, the -300, -400 and -500. The main differences were undated avionics, stretched bodies and new engines. The chosen CFM56 was a high by-pass ratio engine with a larger intake fan. The new engine could no longer be accommodated under the wing because of the original designs ground clearance so a compromise moved the engines forward on a stubby pylon and reduced the diameter of the inlet fan so that the fan blades where still clear of the ground. This was not quite enough as the CFM56 auxiliary gearbox was mounted at the 6 o’clock position under the engine. For the classic aircraft the gearbox was moved to the 9 o’clock position. All this resulted in the engine nacelles of this aircraft range having its distinctive flattened off bottom. Incidentally the 737 Classic acquired another nickname “Hoover” as it often ingested dust from the runways, taxiways and aprons.
So far so good, sales took off and effectively killed off both the DC9 and the 111. But a new challenger had entered the market, Airbus with its A320 family of designs where modern engines and materials meant lower operating costs and Airbus where eating into Boeing’s market. When Boeing lost a long standing customer, United Airlines to the A320, they decided to launch the Next Generation (NG) models. The first was the 737-400 which was rewinged and updated to became the 737NG-800.
The sales war between Boeing and Airbus continues today. Boeing is on to 737 Max family and Airbus onto it A320neo (new engine option) family. Once again Boeing has been playing catch up, in June 2010 they had considered replacing the NG range with a brand new design but had kicked the decision into 2011. However in December 2010 Airbus announced the A320neo. By June 2011 Airbus had picked up 667 commitments for the new model and the straw that broke the camels back seems to have been a massive new order from American Airlines in July. Until then American Airlines had been a Boeing monopoly but 260 of these orders were for Airbus. There was a little hope for Boeing in that American promised a 100 aircraft order to them if a re-engined 737 was announced. In August Boeing caved in and announced the 737 Max range effectively a re-engined NG range using the same CFM LEAP engine as the Airbus A300neo.
Boeing had a problem, the new engine had a bigger fan than the one it was replacing. Previously they had opted for a slightly smaller diameter fan but their calculations indicated that the new Max aircraft would offer a better fuel usage than Airbus only with the standard size fan. Once again Boeing opted to move the engine forward, however this introduced a tendency for the nose of the aircraft to pitch up. If this happened when the aircraft was taking off, it could cause it to stall. This wasn’t too disastrous, but it meant that the plane had different handling characteristics to the older models. This in turn meant that pilots would have to re-qualify on the new aircraft meaning extra training and cost to the customers. Boeing want to keep commonality between the NG and the new Max so they opted for a software solution, it introduced a computer program called Manoeuvring Characteristics Augmentation System (MCAS). This software uses a sensor mounted on the nose of the aircraft to detect its attitude and automatically adjusts the horizontal stabilisers to tilt the nose back down if it predicts a stall. MCAS was not mentioned in the aircraft flight manual and the pilots had no idea of its existence. The only way that a pilot would be aware that the system was installed at all was if the Airline had chosen to install, at extra cost, an optional piece of instrumentation that displayed the aircraft’s angle of attack. With this installed the pilots would have got a warning if the sensor detecting the aircraft attitude failed, but as it wasn’t mentioned in the Flight Manual and they weren’t trained to recognise it they wouldn’t have known what to do! In addition Boeing designers had assumed that all airlines would purchase this option, in fact few did.
Recently there has been two 737 Max accidents where the aircraft have dived into the surface. The first aircraft was operated by Indonesian airline Lion Air when a 737 Max 8 crashed into the Java Sea 12 minutes after take off. The pilots had told Air Traffic Control they were having difficulty controlling the aircraft and sort permission to return before crashing. The aircraft black box indicated that the attitude sensor had failed and that the MCAS had repeatedly tipped the aircraft nose down assuming it was about to stall when it was in fact climbing normally. The Lion Air pilots had no idea what was happening and the system eventually flew the plane into the sea.
Following this accident Boeing were forced by the US Federal Aviation Administration to release information about MCAS and published data on how to disable it in an emergency. Pilots were supposed to memorise a check list and what to do if they had a problem with the system. First they had to operate a switch on either the Pilot or Co-Pilots joystick. If they did nothing more the system turned it self back on 5 seconds later, however if within those 5 seconds they turned off another switch the system was disabled. The pilots should now be able to control the stabilisers by turning hand wheels located on the sides of central console.
The second accident was to an Ethiopian Airlines 737 Max 8 which crash into the ground 6 minutes after taking off from Addis Ababa. Once again the Pilots reported difficulty controlling the plane and asked for permission to return but crashed first. Very quickly satellite data showed the similarities with the Lion Air flight and some countries immediately grounded all 737 Max types. Following analysis of the black boxes it became obvious that the same thing had happened, the attitude sensor had failed and MCAS had taken over repeatedly forcing the nose down. But this is where things were a little different, the Pilots recognised the problem and disabled MCAS. They attempted to manually move the stabilisers but simply didn’t have the muscle power to do so. Why couldn’t they manually control the stabilisers? It seems that they were going too fast and the air pressure on the stabilisers was too much to overcome manually. They had the throttles fully open because they were climbing on take off. In desperation they turn MCAS back on but still crashed.
In April 2019 the Boeing CEO stated that the aircraft was “properly designed” and that the crash in part was caused by the Pilots failing to follow the proper procedures. An American Senator has waded in claiming that this second accident was Pilot error, because they switched MCAS back on. This has been denied by the Chief Executive of Ethiopian Airlines. The Airline believes that the Pilots were in an impossible position. Some of their instruments were not working or were unreliable, alarms were soundings and the stick shaker was operating. They were low to the ground and needed power to climb and if they throttled back they would crash. It was also claimed that simulator training showed Pilots what to do in the event of MCAS problems. This was proven to be correct, however Boeing had to admit that the simulator software failed to replicate the incident properly as the stabiliser trim wheel turned freely unlike in real life.
In the meantime all Boeing Max aircraft remain grounded while Boeing work on a software solution. This is understood to amalgamate the signals from two attitude sensors and to give a warning if one fails or the two are giving wildly different readings. It is in Boeing’s interest to get the software certified as they are building up substantial liabilities. Airline have aircraft they can’t use and are paying Boeing for and replacement aircraft have had to be leased to continue operations. All this is without any legal actions being taken by the relatives of dead passengers and aircrew. Boeing are still building new 737 Max aircraft which they cannot deliver. The FAA who will have to certify the new software are also treading very carefully. It has been suggested that the relationship between Boeing and the FAA is difficult. In the past it is claimed, due to the system operated by the FAA, Boeing virtually self certified aircraft and software.
All this appears to be a result of that original fateful decision to design an aircraft with low ground clearance.
We have not heard the end of this saga, there is sure to be more to come.
© WorthingGooner 2019
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