When the Great War began, a grammar school boy from Newcastle upon Tyne who had gone into business as a shipper and trader in the far reaches of the British Empire, found himself in the skies above Flanders. Aviation was in its infancy, and every flight had an element of the experimental about it.
When Britain declared war on a Germany already marching through Belgium in early August 1914, one Leonard Learmount, aged 25, was employed in the Straits Settlements (Malaya and Singapore), working for London-headquartered shipping and trading company Paterson Simons.
Life in the British Empire’s warmer climes was good for a young single man then, expat clubs providing social connections and sport.
Learmount had also joined the Malay States Volunteer Rifles (MSVR), a British overseas military reserve unit, as a Private Soldier. Nevertheless, following the outbreak of a war predicted to be “over by Christmas”, that November he took a ship back home to join up.
It’s not clear why he was chosen for training as a pilot in the Royal Flying Corps (RFC), but given the indicators for other such personnel choices at the time, it’s probably because the MSVR had trained Learmount to ride and maintain a motorcycle. These skills, combined with his maths and physics education at the Royal Newcastle Grammar School, probably swung the decision.
Learmount reported to Brooklands aerodrome, Surrey, on 19 March 1915 for RFC flying training, and his flying log book says he got airborne the next day for his first lesson in a Maurice Farman “Longhorn” biplane, an ungainly French-designed machine with many of the same basic features as the Wright Flyer.
His instructor, Sgt Watts, hadn’t been trained as an instructor, he merely had flying experience. The RFC didn’t begin formally training instructors until 1917.
Learmount flew his first solo on 2 April, exactly two weeks later, having flown ten trips within sight of the airfield and logged a total of 3h 10min in the air. The day before – April Fool’s Day – he had flown a sortie lasting 45min, by far the longest duration trip he had flown. In the remarks column of his flying log book he wrote: “First time controlled machine from pilot’s seat. Did several landings. No wind – no bumps.”
Leonard’s entire pilot training lasted 12 weeks to the day he was posted, as a 2nd Lieutenant, to No 7 Squadron at Saint Omer, France, about 25km south-east of Calais. He’d accumulated exactly 24h airborne time, and the entry in the “remarks” column of his log book for his 9 June final training sortie reveals how much the RFC was prepared to forgive to get pilots rapidly to the front line. It says: “Pancaked over sheds, smashed undercarriage and one wing landing.”
Estimates of the number of pilot and observer deaths in the Great War have been set as high as 14,000, with 8,000 of them occurring during training. More recent studies, combining fatalities, missing, shot down, and captured suggest 9,000 is closer to the mark for the total, and the number of specific training casualties is uncertain – but it was staggeringly high by today’s standards. A young American aviator training with the RFC at its Montrose, Scotland training base in 1913 wrote home that “there is a crash every day and a funeral every week.” And that was just on his base.
Anyway, the landing at the end of Learmount’s final training sortie was clearly good enough for the RFC, because the next inscription in his log book is: “Arrived in France 12 June 1915.”
Continued tomorrow, Episode 2: Learmount arrives at the RFC aerodrome at Saint Omer, where he learns to fly a new type and to cope with operations in hostile airspace.
On 10 March next week, here, you’ll find the first episode of a series called “Leonard’s War”.
It’s a history of aviation in the Great War of 1914-1918 through the experiences of one man, a Royal Flying Corps pilot, who survived and didn’t think he’d done anything special. But just by surviving nearly three years in the skies over Flanders, he had.
“Leonard” was Acting Major LW Learmount DSO MC RFC, my grandfather, who took command of No 22 Squadron RFC in France less than two years after his first RFC flying lesson. In war, that, too is unremarkable.
There is information here that has not seen the light of day, because it was routine stuff. But routine is what most aviators do most of the time, in peace and in war, so this is a story of military aviation in its experimental years, where more aircrew died during training than in action.
Munich airport has just welcomed a Lufthansa Airbus A350-900 crew from the longest non-stop flight the airline has ever made. The trip also entailed the longest crew-duty period in living memory for the pilots and cabin crew.
On Sunday 31 January the crew of 16 – commanded by Captain Rolf Uzat – took off from Hamburg bound for the Mount Pleasant military airfield on the Falkland Islands. The A350-900 (D-AIXP) covered the 7,392nm (13,700km) distance in 15h 26min.
The 7,230nm return flight on 4 February took 14h 03min, an all-time long-distance record for an incoming flight to Munich airport.
Each of these airborne legs involves formidable crew duty periods, but because of Covid-19 the crew and passengers for this special flight also had to quarantine for two weeks in a Bremerhaven hotel before departure, making the total duty time for the return trip a full 20 days.
The 40 passengers flown from Mount Pleasant to Munich were the crew of the research vessel “Polarstern”, working for the Alfred Wegener Institute’s Helmholtz Centre for Polar and Marine Research (AWI) in Bremerhaven.
Environmental awareness could be embedded in future airline pilot training if a new study by a European training think-tank is accepted at the European Union Aviation Safety Agency (EASA).
EASA’s Aircrew Training Policy Group (ATPG) has produced an advisory paper that takes – as its starting point – the fact that there is no mention anywhere in training syllabi of preparing pilots to operate in an environmentally friendly manner. Entitled “Environmental Awareness Training for Pilots”, the paper points out how incongruous this looks when trade bodies like the International Air Transport Association have, for several years, publicly acknowledged that the industry must strive toward environmental sustainability in the face of accelerating public concern about global warming, atmospheric pollution, noise, and species extinction.
The chief authors of the report – Marina Efthymiou, Assistant Professor in aviation management at Dublin City University, and ATPG chairman Captain Andy O’Shea – point out that, although pilots may be instructed in fuel-saving techniques on command courses, that is not the same as “embedding ecologically friendly flight operations in young pilots’ DNA from their early training.” If EASA were to accept the paper’s argument and develop appropriate changes, they argue that standardising this approach to pilot training – and air traffic management/ATCO training also – would have the potential to influence a way of thinking, and thus to benefit operational behaviour. Efthymiou points out that fuel management training at airline level is not standardised, neither are its results measured. “The purpose here,” she explains, “is sustainability, not saving fuel costs.”
The advisory paper points out: “Traditionally the management of these three decision-based functions (fuel, time, noise) has mostly been considered as solely within the remit of the pilot-in-command.” Now, says the study, the proposed incorporation of environmental awareness into all pilot training is intended to “encourage good behaviour through early, attitude-forming education thereby contributing to the improved environmental aware performance of all pilots.”
O’Shea believes that adopting this proposal need only entail a “re-balancing” of existing training programmes, not radical change, embedding objectives in already-adopted safety instruction concepts like threat and error management (TEM). He suggests that “by recording objective observable behaviour (OB), and TEM outcome data on how recurrent pilots manage environmental scenarios, powerful insights can be generated to help drive a feedback loop into initial type rating training.”
In the end, airlines would benefit financially from the care taken by pilots imbued with a culture of care for their aircraft and the environment, the study argues. Meanwhile, at a time when airlines are spending some of their public relations budget on campaigns to persuade travellers of how ecologically aware they are, and while movements like “Flygskam” (Flight Shaming) are competing for passengers’ attention, being able to claim – truthfully – that “your pilots” are trained to care about their skies might also prove a marketing advantage.
The European Union Aviation Safety Agency (EASA) has published a Proposed Airworthiness Directive (PAD) , signalling its intention to approve the Boeing 737 MAX’s return to Europe’s skies “within a matter of weeks” – probably about mid-January.
But Europe is specifying a few requirements that the US Federal Aviation Administration (FAA) has not demanded.
It was on 20 November that the FAA approved the aircraft’s return to America’s skies, but US carriers have many preparations to complete before resuming commercial services with the Max. American Airlines reckons it will be ready by the end of December.
EASA, however, wants to see the application of some operational measures that the FAA does not require. It insists, nevertheless, that the Max airframes in America and in Europe will be the same. The agency explains: “The [PAD] requires the same changes to the aircraft as the FAA, meaning that there will be no software or technical differences between the aircraft operated by the United States operators and by the EASA member states operators.”
The EASA PAD is a consultation document, and all responses have to be received by 22 December. EASA executive director Patrick Ky is at pains to point out that the agency, while cooperating with the FAA on correcting the anomalies in the Max’s manoeuvring characteristics augmentation system (MCAS) (see immediately preceding blog entry), insisted on looking independently at the whole issue.
Ky explained: “EASA’s review of the 737 MAX began with the MCAS but went far beyond. We took a decision early on to review the entire flight control system and gradually broadened our assessment to include all aspects of design which could influence how the flight controls operated. This led, for example, to a deeper study of the wiring installation, which resulted in a change that is now also mandated in the [PAD].” That, basically, is a requirement to bring the venerable 737’s design up to date, and is a signal that the days of “grandfather rights” – a dispensation to build the 737 Max as earlier versions of the 737 were constructed rather than as new aircraft have to be designed – are numbered.
The Max airframe design came through all the handling tests satisfactorily, as Ky explained: “We also pushed the aircraft to its limits during flight tests, assessed the behaviour of the aircraft in failure scenarios, and could confirm that the aircraft is stable and has no tendency to pitch-up even without the MCAS.”
Two principle differences between the FAA and EASA requirements are explained as follows: “EASA explicitly allows flight crews to intervene to stop a stick-shaker from continuing to vibrate once it has been erroneously activated by the system, to prevent this distracting the crew. EASA also, for the time being, mandates that the aircraft’s autopilot should not be used for certain types of high-precision landings [and approaches such as RNP-AR]. The latter is expected to be a short-term restriction.”
The crew intervention mentioned would allow the pilots to pull the stick-shaker circuit breaker. The stick-shaker – a system designed to alert pilots to an approaching stall – was one of the distractions that faced the Lion Air and Ethiopian Airlines crews before they lost control of their aircraft, despite the fact that the shaker was triggered by a false warning.
The FAA doesn’t see the need for this intervention, because the modifications have ensured that a single sensor failure will not trigger the stick-shaker any more.
Boeing and EASA say they have agreed to continue tests to see if they can further strengthen the aircraft’s systems’ resilience to angle of attack (AoA) sensor failures – the causal trigger for the two fatal Max accidents, and Boeing has also made this promise: “Boeing will also conduct a complementary Human Factor assessment of its crew alerting systems within the next 12 months, with the aim of potentially upgrading these to a more modern design approach.”
After nearly two years of grounding, Boeing’s 737 Max series has been cleared by the US Federal Aviation Administration to carry fare-paying passengers once again.
This is the first step in a redemption process for one of the world’s truly great engineering companies. Like a boxer who dropped his guard for just a second, Boeing has taken a punch that has knocked it to the canvas, and the referee had started counting.
Now, air traveller reaction is nervously awaited. Will the public believe claims by the FAA and Boeing that, together, they have confined to history the flaws that caused the 737 Max fatal crashes in 2018 and 2019?
The FAA – blamed along with the manufacturer for the lapses in design oversight that led to the two accidents – has declared the aircraft safe to operate in America. One by one, other national aviation authorities (NAA) are expected to follow suit.
Oversight of the type’s rehabilitation continues to be the FAA’s responsibility, but decisions on the systems and software changes applied to the Max have been made by multinational teams. Bodies formed to decide what changes were needed – and then to see them implemented – included the Joint Authorities Technical Review (JATR) representing nine nations plus the European Union Aviation Safety Agency (EASA) – and the Joint Operations Evaluation Board.
The relationship between the FAA and Boeing was much criticised in the accident investigations and the JATR review process . For that reason, the reaction of EASA to the Max’s clearance to fly is seen as critical.
Not only is EASA the agency that oversees safety in the region containing the largest group of aerospace industries outside America, but its contribution to the JATR recommendations made clear EASA was not happy with the FAA’s former piecemeal approach to certifying critical changes applied to the 737 Max.
Its opprobrium was directed particularly at the FAA’s approval of the flawed Manoeuvring Characteristics Augmentation System (MCAS), unique to the Max, and not used in earlier marques of 737. It recommended “a comprehensive integrated system-level analysis” of the MCAS, and of its integration into the total system-of-systems that constitutes a modern aircraft (for more detail, see “The Failures and the Fixes”section following this article).
So it was with heartfelt relief that Boeing heard EASA’s executive director, Patrick Ky, report on Max progress to the European Parliament Transport Committee on 29 October. Ky told them: “We are fully confident that, given all the work that has been performed, and the assessments which have been done, the aircraft can be returned safely to service.” Ky’s statement suggests EASA will re-certificate the 737 Max in Europe soon after the FAA’s announcement.
Meanwhile, out in the real world, Covid-19’s near-immobilisation of commercial air transport worldwide has rendered the Max’s long grounding almost invisible to the media and the public. Because of the far lower level of air travel activity, the airlines have been able to live without the 387 Maxes already delivered to them, and also without the additional 450 that have rolled off Boeing’s Renton, Washington production line since then. The latter are all in storage, awaiting any updates not already incorporated, and ultimate delivery.
Although clearance to fly has now been delivered, even in the USA the airlines will not instantly be re-launching their already-owned 737 Max fleets. The status of all the proposed software and hardware modifications to the type will not have been confirmed until the moment the FAA signs it all off.
American Airlines has said it hopes to start getting its Max fleet airborne before the end of December.
Once the FAA has done that, getting the Max fleet ready for the sky will be an aircraft-by-aircraft, crew-by-crew process. In many airframes, a knowledge of what changes were coming has enabled a great deal of the work to be done. But also, because of the hardware and software changes to the Max, the crews have to be trained to use the new systems.
Incidentally, while the Max series was grounded, the FAA decided to order some additional modifications – completely unrelated to the crashes – to bring the type fully in line with modern safety regulations. For example, one of these involves the re-routeing and separation of wiring looms that the 737 had previously been allowed to sidestep under “grandfather” rules.
The number of lessons for manufacturers and regulators to learn from this aerospace drama is legion.
The failures and the fixes
Just a reminder: the 737 Max series fleet was grounded in March last year as a result of findings from the investigations into to the Lion Air and Ethiopian Airlines fatal crashes, respectively in October 2018 and March 2019.
The primary causal factor of the Lion Air Max crash was erroneous triggering of its manoeuvring characteristics augmentation system (MCAS) by a faulty angle of attack (AoA) sensor, according to the Indonesian final accident report. It is at the MCAS that Boeing’s corrective efforts have mostly been directed.
In both the accidents, the aircraft’s AoA sensor that feeds data to the MCAS wrongly indicated a very high AoA soon after take-off. The system reacted by providing nose-down stabilizer rotation that took the pilots by surprise. They did not understand the reason it kicked in, and their efforts to reverse the strong nose-down pitch did not succeed. Both these events occurred soon after take-off, and because the MCAS kept repeating the nose-down stabilizer in response to the continued erroneous high AoA sensor signal, the loss of height quickly resulted in impact with the surface.
During the examination of all the issues arising from the accidents, the JOEB was aware there were solutions to the situation in which the crews found themselves. But the fact that two crews in different regions of the world were so confused by what the MCAS was doing that they lost control had totally eclipsed pilot failings as the main issue.
MCAS was designed to trigger only in a specific flight configuration that causes the Max’s centre of lift to move slightly further forward, delivering a slight nose-up moment that can be countered by flight controls. This configuration is a combination of relatively low airspeed, flaps up, with the aircraft being flown manually. In the case of the Lion Air and Ethiopian flights, the pilots decided to continue to fly the aircraft manually during the early climb, rather than engaging the autopilot, so this precise flight configuration was encountered as soon as the flaps were fully retracted.
With flaps up, and still at a fairly low airspeed, the aircraft would be at a high angle of attack, and not far above the stall. FAA regulations require that, in the proximity to the stall, one of the “feel” cues to the pilots is that there should be a linear increase in the required control column force versus elevator displacement response, but the Max’s aerodynamics in this configuration had negated this effect, and MCAS was designed to restore that pilot cue automatically.
The JATR decided that MCAS’ fatal design weakness, above all, was that it was triggered by a single AoA sensor with no backup in case the unit had a fault or suffered damage. It seems Boeing and the FAA had overlooked that possibility, and had not explored the potential effects of erroneous inputs. Their excuse at the time was that the system was not seen as a critical one, rather as a refinement.
The 737 Max had always been fitted with two AoA vanes, but originally only one was wired up to MCAS, and there was no flight deck indication of a disparity between the two sensors if a difference developed, which could have warned the pilots of a potential vane fault.
The hardware fix agreed by the JATR was that both AoA sensors would now feed into the MCAS, there would be an automatic comparison between them, and if there was more than a small disparity the MCAS would be locked out completely, because the aircraft can be flown without it.
The software fix also ensures that – now – the MCAS only operates once per high AoA event, so the repeated nose-down pitch demand by the stabilizers that led to the two accidents would not occur. In addition, the two flight control computers (FCC) now continuously cross-monitor each other.
After the hardware and software changes, the final improvements – overseen by the multinational JOEB – are to pilot training and cockpit drills for the Max series.
Now, even if the pilots are coming to the Max from the very similar 737NG series, pilots must undergo a one-off training session in a Max full flight simulator. This involves recovery from a full stall, dealing with a runaway stabilizer, practice manual trimming at high speeds (and therefore high trim loads), and crew cooperation on all these exercises.
Non-normal checklists have now been compeletely revised, and contain updated procedures that concentrate particularly on the operation of the horizontal stabilisers and trim controls, both in normal operation and in the case of all potential faults. The drills deal with runaway stabilizer, speed-trim failure, stabilizer out of trim, stabilizer trim inoperative, airspeed unreliable, altitude disagree, and AoA disagree.
Computer based training (CBT), containing video of crew exercises using the real controls, teaches drills for the following: airspeed unreliable, runaway stabilizer, the speed trim system, trim controls, and differences between the autopilot flight director system (AFDS) in the NG series and the Max series.
Testing the changes
Boeing and the FAA say they have put in 391,000 engineering and test hours developing the solutions, which have then been tried for 1,847 hours in simulators and for 3,000 airborne hours in the real aircraft.
Very soon – perhaps this week – the US Federal Aviation Administration is expected to declare Boeing’s 737 Max safe to fly again in America’s skies, lifting nearly two years of compulsory grounding.
Such an event would normally be a subject of press fanfare, but Covid-19’s near-immobilisation of commercial air transport activity worldwide has rendered the Max’s long grounding almost invisible to the non-specialist media and the public.
The airlines have been able to work not only without the 387 Maxes already delivered, but without the additional 450 that have rolled off Boeing’s Renton, Washington production line since then – only to be delivered straight into desert storage.
The changes being applied – at the FAA’s behest – to this latest version of the highly successful 737 series are partly to correct design flaws that allowed two notorious fatal crashes to occur, but some additional modifications will bring the type fully in line with modern safety regulations that this marque had previously been permitted to avoid under “grandfather rights”.
Once the Max fleet had been grounded, it made sense to incorporate not only the changes required to make it safe, but also improvements that would prolong the marque’s commercial desirability for as long as possible. That is essential because Boeing’s next product in this market sector will be entirely new, and will not be launched for some years.
The truth is that the 737 line has reached the end of its viable development life, but given the fact that it has been in continuous production since 1966 through four iterations, that should not be too surprising.
Basically, the Max marque was intended as a stop-gap while Boeing came up with a “new mid-market airplane”, but when the Max hit the marketplace it was astoundingly successful. Its price was right, its economics excellent, its delivery guaranteed, and it was a known and trusted quantity. And all this despite the fact that it is an old fashioned, mechanically controlled machine surrounded by digitally controlled competition.
This relaunch of the Max into an airline world decimated by Covid-19 is going to be watched with bated breath, not just by Boeing, but by the whole industry.
Public perception of the aircraft is key. Will they see it as safe? Will it be safe?
As soon as the FAA announces the detail of its decision, the answers will be here.
This man did not die for his country. He just came within a whisker of doing so countless times between June 1915 and March 1918 when he was flying over the hellish battle lines of the Western Front in the Great War.
Leonard Learmount is not listed as an ace, but he was an RFC pilot and squadron commander. When I, as his grandson, began researching his military life, I discovered a man who had been a businessman in the Far East before the war, and returned to the same business after it in 1919. He kept no records of his military flying and never talked of it, but clearly retained a love of flying, because he founded flying clubs that still exist in Kuala Lumpur and Singapore.
As this entry in the RAF Museum’s blog points out, his dogged persistence as a multi-role aviator for nearly three years over the front line, facing high risk every mission and being wounded in action twice, is as much a representation of the spirit of the RFC and RAF as the stories of the aces.
His story, and that of his squadron – No 22 – are told in more detail in the Summer 2020 edition of Cross & Cockade International, the quarterly journal of the First World War Aviation Historical Society. For anyone interested in the history of aviation – indeed the origins of aviation – and history of the Great War, I cannot recommend the Society highly enough. Membership doesn’t break the bank.
Having researched the detail of a specific low-altitude photo-reconnaissance sortie Learmount flew over the Hindenburg Line on 10 May 1917, I commissioned aviation artist Tim O’Brien to paint the scene of the preparation for departure. The return from the mission was more messy, because the aeroplane had been shot-up and Learmount wounded. To get clear photographs of the enemy lines the pilot had to fly the aircraft so low it was within easy range of small-arms fire, let alone “archie” – anti-aircraft fire. And the flying had to be steady, making the aeroplane a sitting duck. But they got the photos back to base, and their quality was high, rendering vital information about enemy readiness states.
Today I was replying to a message from a good friend in Maryland, and found that I’d written to him what I have wanted to put up here for a while.
He had picked up on something I wrote in FlightGlobal/Flight International a month or so ago about Boeing CEO Dave Calhoun’s thoughts on the kind of control interface that would be best for pilots flying Boeing’s next clean-sheet-of-paper aeroplane in tomorrow’s skies.
This is what I suggested to him:
“I don’t actually know what Boeing will do with the pilot’s “joystick” or yoke equivalent in its next-generation aircraft. My observation that you picked up on was based entirely on the musings of Boeing’s new boss Dave Calhoun when he suggested they might need to have to do a complete re-think of how to tomorrow’s pilots should interface with tomorrow’s aeroplanes.
The thing about the airline piloting job now is that it has drastically changed. Even aircraft originally designed in the 1960s, like the 737 series, in their latest versions put just as many computers between the pilots and the flying control surfaces as Airbus does with its FBW fleet. So any remaining efforts to fool the pilots into believing that the control feedback they feel is the real thing is just artifice. And like any part of the system, the artificial feedback can fail and thus mislead.
The only aeroplanes in which Bob-Hoover type stick-and-rudder skills were ever really needed is manually controlled aerobatic machines flown in perfect VMC during a display. Modern combat aircraft, built for aerodynamic instability so as to be manoeuvrable, have had FBW sidesticks for decades, and the pilot’s main task is to direct the mission and its defence, not to use finely-honed skills to keep it flying. The stick is just a device for pointing such an aircraft where you want it to go.
In an airliner you were never supposed to handle it as if you were Bob Hoover flying a display. Nowadays, if you have to fly it manually at all – and 99% of the time you are told not to – your job is ABSOLUTELY NOT to fly it by the seat of your pants, it’s to select the attitude/power combination you need to get you elegantly from where you are to where your passengers wish to go. I can tell you from experience, a spring-loaded sidestick is an easier device than a yoke for selecting an attitude, and as for selecting power, throttle levers do the same everywhere, back-driven or not.
So I’d guess Boeing probably will go down that track. Pilots who still need to be flattered by being presented with controls that look like the old fashioned ones but do not work like them are no longer in the right job!”
No pilot/aeroplane interface is perfect. But choosing the best one for the next Boeing is going to be an interesting job for Calhoun.
Early in 1920 the UK Air Ministry decided that, with an average of 12 air movements a day, the air traffic at London’s main airport – Croydon – needed organising.
The ministry had no template for such a task, but issued a specification for a building they believed would do the job. It was to be called an aerodrome control tower, and the working part of it was to be “15ft above ground level, with large windows to be placed on all four walls”.
Radio communication was already in use, but even primitive radar would not be developed for another 20 years.
Radio direction-finding, however, provided the Civil Aviation Traffic Officers (CATOs) with the bearing from the airport of any aircraft transmitting a radio message, thus they could provide the crew with a course to fly to arrive overhead the aerodrome. They could also provide the pilots with weather information, including visibility, wind speed and direction, but also the approximate position of other traffic in the area so the crew could keep their eyes out for it.
Navigation was primitive in aviation’s early years. Clearly identifying the destination aerodrome so the crew landed at the right one was important. The pilots were helped to find the aerodrome by a bright, strobing “lighthouse” beam – green alternating with white – which was located on a high point. When control towers came in, the light was above the tower.
Positive airfield identification was provided by very large lettering spelling out the airport name, either on the ground, or on the roof of a large hangar.
Separation between aircraft, if there was more than one near the aerodrome at any time, was assured visually by pilots looking out for other aeroplanes, with advice from the tower if necessary as to the position of potentially conflicting traffic.
Protocols about which of any two aircraft has the right to hold course and which should give way are set in the rules of the air, similar to the rules which mariners follow on the sea, and a disciplined circuit pattern over an aerodrome was a system with which pilots were familiar.
Permission to land or take off could be signalled by radio, or by a CATO shining a green aldis lamp toward the aircraft cockpit. Similarly, a red lamp would refuse permission. Firing off a green or red Verey flare from the tower was an alternative.