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Ensuring that the Materials and Workmanship of British Aircraft Comply with the Air Ministry Standards


THE ENGINE as well as the airframe is carefully examined when a Certificate of Airworthiness is to be renewed











THE ENGINE as well as the airframe is carefully examined when a Certificate of Airworthiness is to be renewed. Any worn or defective part must be replaced. In this illustration engineers are seen checking over the oiling system of a Gipsy engine after a complete overhaul. It is largely because of the stringent regulations governing the issue of Certificates of Airworthiness that modern aero engines are so reliable.













AIRCRAFT registration marks (see the chapter on “Aircraft Markings”) are of far greater significance than the number plates of a motor car. To a certain extent their functions are similar, as both facilitate identification; but aircraft registration marks are, in addition, the outward indication that the aircraft has passed tests to ensure that it is airworthy.


The letters G-AZZZ on an aeroplane not only enable the various authorities to keep track of it, but they are also a large duplicate of the number on that particular machine’s file, which is always accessible at the Air Ministry, London, and which is continually amended and brought up to date. If at any time G-AZZZ were fitted with a new engine, extra wireless equipment, or even an additional navigation instrument, that fact would be recorded in the file marked G-AZZZ. Behind all this is the phrase “Certificate of Airworthiness”, more generally referred to as “C of A”. Before any machine is allowed to fly in Great Britain the Air Ministry must be satisfied that it conforms to certain standards. The various papers in the files testify that the requirements have been adhered to, and the large letters on the machine form the last link in the system.


In the beginning of aeroplane history there was no such system in operation. Anyone who had the inclination and the money to build a flying machine could do so, and the machine could then be flown without restraint, at any time, and the pilot could carry passengers in it if he wished. There was no supervision. No standards were set for the constructor to follow. In the event of an accident an inquest often ended the affair, and no real inquiry was made into the cause. The first advance was made in the beginning of 1914, when Captain Geoffrey de Havilland was appointed by the Government to the new post of Inspector of Aeroplanes. It was then his job to examine different makes of aeroplane, to fly them and to report on their merits and shortcomings. A few months later war broke out, and it became necessary to standardize production, or rather to sort out the good from the bad. There was a sudden demand for aeroplanes, and only the better designs were chosen. Mass production followed, and against this the independent manufacturer had little chance. The obvious requirements of a war machine set the pace, and increasingly stringent demands set their own standards.


EVERY DETAIL OF AN AEROPLANE is checked before a Certificate of Airworthiness is issued by the Air Ministry






EVERY DETAIL OF AN AEROPLANE is checked before a Certificate of Airworthiness is issued by the Air Ministry. This illustration shows engineers checking the trueness of the airframe of an aircraft by means of a plumb-line. The first machine of a new design is known as a “Type Aircraft”. Other machines built to the same design are called “Subsequent Aircraft” and do not have to go through certain special tests before being given a Certificate of Airworthiness.








After the war and before the aviation industry was thoroughly reorganized, men in authority began to consider seriously the need for controlling air transport. Something had to be done quickly to make sure that the industry did not become unmanageable. It was not sufficient for pilots to pass certain qualifying tests, but the machines which they would fly must also have been proved safe and suitable. To ensure this it was necessary to subject to regulation manufacturers as well as pilots. In 1920 the Air Navigation Act was passed - “An Act to enable effect to be given to a Convention for regulating Air Navigation, and to make further provision for the control and regulation of aviation”.


A Convention had been signed in Paris on October 13, 1919, for the purpose of arriving at certain uniform rules about international air navigation, and the 1920 Act was passed to bring British enterprise into the plan. Among other details of the Act, provision was made for the licensing, inspection and regulation of aerodromes, for access to aerodromes and aircraft landing places, and for access to aircraft factories for the purpose of inspecting work in progress. The Act provided also for the prohibition or regulation of unlicensed aerodromes and for the licensing of inspectors of aircraft.


The Act further specified the conditions of the issue, renewal, endorsement, suspension and cancellation of certificates or licences, and dealt with the examinations and tests to be undergone. Other sections of the Act were concerned with the register of British aircraft, and with the conditions in which aircraft might be used for carrying goods, mail and passengers. The full wisdom of the Act can be appreciated today, with aviation in its present state of advancement. Had there been no control, there could have been no guaranteed standards of safety for air travellers. In 1920 many people thought that the conditions governing manufacturer and flight were too strict; but it was there that the Act showed its first usefulness; it encouraged the right type of development.


Fixed Safety Standards


In anticipation of the Act, and recognizing the need for reasonable standards, several manufacturers began to reorganize their methods.


Today, no matter where an aeroplane has been made, it must not be flown in the British Isles unless it has been granted a British Certificate of Airworthiness, or unless its foreign “C of A” has been approved by the British authorities. The “C of A” is a document which proclaims the machine as fit to fly, and it has to be carried in the machine wherever it goes.


There is no law to prevent anyone from building an aeroplane in Great Britain, but no one, not even a fully licensed pilot, may fly that aeroplane until it has been certified as airworthy by a responsible official. The only exception to this rule is that a flight may be made for the purpose of testing the machine for the certificate. This rule applies as strictly to the largest factory as it does to the private individual.


THE UPPER COMPONENT of the Short-Mayo composite aircraft Mercury, undergoing flight trials at RochesterDistinctions are made between private, or amateur enterprise, and well-established factories; but such distinctions apply only in so far as the Secretary of State will accept certain statements from a firm of repute. Any private individual requiring a “C of A” for a machine of his own ideas and construction must first of all apply to the Air Ministry for an application form, and he must supply full details of the proposed machine - that is, drawings and plans showing the method of construction.





THE UPPER COMPONENT of the Short-Mayo composite aircraft Mercury, undergoing flight trials at Rochester, Kent, where it was built. Every aircraft has to be passed by Air Ministry pilots before a Certificate of Airworthiness is granted. After certain flight tests have been completed the aircraft has to undergo full-load tests. On its tests the Mercury carried 1,200 gallons of petrol for the full-load separation trails from Maia, the lower component of the Short-Mayo composite aircraft. After the separation 1,000 gallons of petrol had to be jettisoned to permit the Mercury to alight with a light load. Thus the jettisoning valves and the inter-compartment valves of the petrol tank - which runs through the wings - were tested at the same time during the trials.





In all probability the Secretary of State will then ask for fully dimensioned sketches in duplicate of various parts, and these will be thoroughly checked by Air Ministry authorities. Before any such checking is begun, the Air Ministry must be satisfied that certain safety standards have been followed. In other words, unless the constructor knows his job the Air Ministry will not even consider his plans - a reasonable provision, calculated to save valuable time and work.


Next, assuming that the design is on the right lines, but does not conform exactly with the fixed standards of safety, the constructor will probably be asked to furnish evidence that it is sound; this will necessitate wind tunnel tests. Either the constructor will be asked to have these tests made, or the Air Ministry will make them at the applicant’s expense.


If the plans are passed, the constructor may carry on with the work; but he must keep in close touch with the Air Ministry and adhere absolutely to his plans. Should he make modifications, or should the Air Ministry require modifications, these must all be tested and passed as before. No construction work can be undertaken until the particular plans for that part of the work have been passed. Not only the plans, but also the materials, must be approved before assembly.


During the building of the aircraft the constructor must allow Air Ministry inspectors full access to all plans, materials and plant, and he must satisfy them that work is being carried out in accordance with his original design, and to a reasonable standard of efficiency. If he buys a part from any outside source, that part must bear the stamp of an Air Ministry inspector.


When the machine has been completed it must be flown in the presence of a representative of the Secretary of State, who will then forward to the Air Ministry a full report on its behaviour. After that the constructor must be prepared to hand over the machine to the Secretary of State for its official flying trials. These trials are generally carried out at Martlesham Heath, Suffolk, by he Government test pilots (see the chapter on “Work of the Test Pilot”). If the machine fails under the tests, no “C of A” will be granted. Should it be damaged or crashed, the constructor must begin all over again.


In an approved aircraft factory, the procedure is more or less the same for a new type, but all later machines of that type receive a “C of A” after less of rigorous tests. The designer submits his plans, with sufficient data to assure the Secretary of State that the prescribed safety standards have been followed. Construction follows automatically as the plans are approved.


The preliminary flying trials are carried out by the firm’s test pilot, reports are submitted to the Air Ministry, and the “C of A” is granted for that machine, as a type, if it passes its “Martlesham” tests in a satisfactory manner.


ALL AEROPLANES used for commercial purposes have to be inspected and passed each day by a qualified ground engineerThe phrase “Type Aircraft” is significant. It means that the factory can proceed, putting that proved type into full production as soon as the first “C of A” has been granted, provided that every machine built to type is identical with the original. All deviations from the original must have the full approval of the Air Ministry.


All machines built to the proved type are known as “Subsequent Aircraft” and Certificates of Airworthiness are granted for each machine upon request. There are needed only the customary test flights by the firm’s test pilot, his report and the final acceptance of the machine by an Air Ministry inspector.





ALL AEROPLANES used for commercial purposes have to be inspected and passed each day by a qualified ground engineer. Any small part that may be replaced - even a nut or split pin - must be one that has been passed by an Air Ministry inspector. Not only is the construction of such small parts checked, but the material of which they are made is also under the jurisdiction of the inspectors.





To qualify for a “C of A”, every machine must be fitted with whatever instruments or gauges that the Air Ministry considers necessary. Such instruments vary with the type of machine and with the work for which it is designed, but they always include an air speed indicator, an altimeter and a revolution counter. For a large passenger machine, gauges for oil temperature and pressure will in all probability be considered necessary, and many gauges must be duplicated in a multi-engined machine. An aeroplane to be used for night-flying must be fitted with instrument lights and an electrical system.


The cost of a “C of A” increases with the size of the aircraft. For example, the Tiger Moth, with a tare weight of about 1,100 lb., cost its manufacturers £60 for the original “C of A”. On the other hand, the huge Empire flying boat’s certificate cost about £300. The minimum charge for a “C of A” is £25 - for a tare weight not exceeding 500 lb. These figures refer to “type aircraft”; certificates for all “subsequent aircraft” cost £5 5s. Only.


A “C of A” holds good for one year from the date of issue, provided that any alterations or modifications to the machine in the meantime have been approved by the Air Ministry. After that the renewal fee is £5 5s. a year for each machine.


So far as British machines are concerned, a strict supervision is thus exercised, both during construction and while they are flying; with imported machines, however, it is obvious that the same supervision cannot be carried out. This difficulty is overcome in two ways.


Most countries belong to a body known as the I.C.A.N. (International Commission of Air Navigation), which lays down certain minimum requirements to be followed by aircraft manufacturers. Although such requirements are not necessarily so stringent as some of those operating in Great Britain, they are more or less the same. In this way a certain desirable standard of workmanship is guaranteed virtually throughout the world.


The United States, Italy and Germany have an arrangement with Great Britain whereby the “Cs of A” of those countries are automatically ratified by the British authorities. This does not mean that an American, Italian or German machine automatically receives a British “C of A” - only a machine built in Britain can earn that - but it means that the foreign “C of A” in question holds good in Great Britain and that the machine is deemed to be good and airworthy.


Air Registration Board


Should any other country wish to export aeroplanes to Great Britain, it would first of all be necessary for the Air Ministry to satisfy itself that production methods and materials were up to standard. Although the foreign “C of A” would have to conform to I.C.A.N. requirements, the authorities in Great Britain insist on a first-hand inspection. Before the “C of A”, were validated, therefore, it would be necessary for someone in authority to visit the foreign country and to examine thoroughly the methods of manufacture,

the class of workmanship and the materials used.


REPLACEMENT PARTS for aircraft in the store of Brook-lands Aviation LtdIt is a big step from the days of 1914, when the post of Inspector of Aeroplanes was created, to the highly organized department, known as the A.I.D. (Aircraft Inspection Directorate) of today. Captain Geoffrey de Havilland had to test and fly odd aeroplanes; the A.I.D. now has to supervise the construction and maintenance of many hundreds.





REPLACEMENT PARTS for aircraft in the store of Brook-lands Aviation Ltd. Each part has an Air Ministry release note number and is stamped with identification data. Should any part prove faulty in use, it is thus possible to trace it back to the department in which it was made and to find out by whom it was passed.





A new department has recently been created to look after British Certificates of Airworthiness. It is known as the Air Registration Board and is designed to relieve the work of the A.I.D. Representatives of the Air Registration Board now visit factories where production of “subsequent aircraft” calls for more or less continuous inspections for “Cs of A”, and the full responsibility of granting certificates and registration is rapidly being transferred. Meanwhile, however, the A.I.D. continues to expand in conformity with the growing aircraft industry. As not only plans of aircraft, but also all materials, have to be approved and passed by the Air Ministry before assembly may begin, the work of the A.I.D. is necessarily onerous. In the early days of supervision it was a simple matter to keep a check on all that was going on; today hundreds of skilled men have to be employed for this purpose.


The process of supervision has been greatly simplified by the nomination of “approved firms”; this means that the Air Ministry keeps a list of aircraft-manufacturing firms whose reputation is above reproach, and installs one or more inspectors permanently on the premises.


These inspectors have been thoroughly trained to recognize the standards laid down by law, and they are capable of testing and passing all parts and accessories produced by the factory. As assembly progresses, their responsibility is also to see that all work is in accordance with the original designs.


As few, if any, aircraft factories build a complete aeroplane from a range of raw materials, the industry is largely dependent upon the good quality of materials and parts manufactured by outside firms; here again the Air Ministry exerts its authority. Before any factory can manufacture, any component for use in aircraft, it must earn the approval of the Air Ministry and be included in the list of “approved firms”.


This restriction applies just as strictly to a manufacturer of split pins as it does to a maker of wheels or carburettors. If the quantity of components produced is high enough, an A.I.D. inspector is installed in the factory to pass materials and test finished products in bulk. Such inspection is not carried out to such limits as to hinder production; for instance, it is sufficient to “pass” a type of rivet, provided that the machine turning out the rivets is satisfactory, and provided the raw material is up to standard. Inspection here would merely cover occasional rivets picked at random from a batch; but a flaw, or deficiency, would be sufficient reason for condemning the batch.


No Weak Spots


The expression Certificate of Airworthiness means that the machine to which the certificate is granted is airworthy in every way and that it is built to a high safety standard. To guarantee that the certificate is unexceptionable, every single detail of construction and design, from the glue, paint and screws to the most vital engine part, must be up to standard in every way. That was

the idea in the beginning, and it has been maintained to the letter ever since. No weak spots can be tolerated in an aircraft.


MANUFACTURERS OF AIRCRAFT and of aircraft components have their own inspection and test staff



MANUFACTURERS OF AIRCRAFT and of aircraft components have their own inspection and test staff who work independently of the Air Ministry inspectors. This photograph, taken at the De Havilland Works, shows a device for checking the quality and hardness of metal parts. The operator is examining a gudgeon pin through a microscope.





To give some idea of the details to be watched by A.I.D. inspectors, it may help to reproduce a few extracts of the notices issued by the Air Ministry from time to time. “Approved aircraft firms”, according to one notice, may apply to the Secretary of State, asking that their reports concerning designs and airworthiness be accepted. The Air Ministry is ready to take the designer’s word that his plans are up to standard, provided that certain agreements are made and carried out, such as:


“The organization of the firm shall be such as to ensure full and proper control of, and coordination between, the design, stressing and drawing offices and the works”, and


“The firm shall employ a test pilot (or pilots) whose flying experience is considered adequate and satisfactory by the Secretary of State.”


Another notice deals with the construction of a radio aerial weight. “ . . . The ends of the cable to be spliced. Splicing must be clean and free from projecting ends. Cover with two turns of linen fabric, and whip with two layers of linen thread No. 12, waxed ...” Yet another notice indicates the care given to engines: “An endurance test of 50 hours is then to be run in five non-stop periods of ten hours each. The first twenty hours of this test must be run on an approved form of brake at nine-tenths of the international power, and at international r.p.m. . . . Twenty hours of the test must be run as a thrust test. . . . Nine hours of the last ten hours’ period must be run at full throttle on an approved form of brake, at the calculated takeoff speed. ...”


A typical A.I.D. notice, of the kind sent out to every licensed engineer and Air Ministry inspector whenever some point arises that has not been covered by previous instructions, deals with the “inspection of duralumin and other high tensile aluminium alloys, such as Y alloy.” According to this notice, the inspection “must ensure (a) that an agreed percentage of the melts be checked for composition by analysis, (d) that sawn billets are free from surface defects, (e) that no material which is unsatisfactory at any one stage is passed on to the next stage.”


The notice incorporates the reminder that “accepted stampings, forgings, tubes, bars, sections, etc., shall be stamped with the specification number, maker’s identification mark, and approved inspector’s stamp.” Parts which are too small to stamp individually “may be parcelled, and the identification marks stamped on a metal tag, securely attached to the parcel.”


A good example of the precise supervision of the Air Ministry over all sections of the industry is found in a recent Notice to Aircraft Owners and Ground Engineers. It refers to airscrew hubs on a named type of engine.


“1. A failure of a wooden airscrew fitted to an aeroplane with a ... engine has occurred which is attributed to the airscrew boss being loose in the hub.


“2. No further flying is to be carried out on aeroplanes fitted with engines of this type until the modifications detailed .... below have been embodied.


“(1) The length of the ⅜" B.S.F. thread on the six hub bolts is to be increased from 0·5 inches to 0·75 inches. Replacement bolts, if required, are to be similar in every way to the existing bolts but with a longer thread, and are to be made of steel to B.S. specification S.11 (or an equivalent specification) and cadmium plated.


“(2) The nuts are to be locked by means of tab washers of mild steel to A.G.S. 194, Part No. 6. These tab washers replace the existing plain washers. ...”


This notice shows clearly how the performance of any aircraft is watched, from the time it leaves the factory as a type until it grows out of date. In the original drawings and tests, the fault referred to was not detected because, in spite of the most careful supervision, only practice can show up certain weaknesses. As soon as any fault appears, however, the Ground Engineer who discovers it during normal routine inspections reports it to the Air Ministry. The Air Ministry immediately investigates it, decides upon a suitable modification and circularizes the details to everyone whom it may affect.


This instance is also a good example of the reasonable latitude which is allowed. It would not be wise to allow a single fault to hold up the activities of every other machine of the type in question. As, however, prevention is better than cure, the Air Ministry insists that temporary measures, at least, should be taken. For instance, a fourth paragraph in the notice states:


“As a temporary expedient .... additional washers, similar to those already in use, are to be fitted under the nuts of the hub bolts, so as to ensure that the nuts may be tightened sufficiently to give the required friction drive . . .”


The notice concludes wFLYING TESTS of new designs of aeroplanes are generally conducted at Martlesham Heathith the following warning: “Certificates of Airworthiness of aeroplanes affected will be liable to suspension or cancellation if the requirements have not been complied with within the period stated. Certificates of Airworthiness will not be renewed and Ground Engineers must not sign Daily Certificates of Safety for Flight in respect of such aeroplanes after the expiration of this period.” Notices of this type, covering every subject, from the floats of carburettors, to the stitching of fabric, have been issued in their hundreds. They are, in effect, bulletins which keep every section of the aircraft industry alive to the changes, developments, modifications and warnings that are decided, upon at headquarters.






FLYING TESTS of new designs of aeroplanes are generally conducted at Martlesham Heath, Suffolk, by Government test pilots. If the machine does not pass certain tests a Certificate of Airworthiness will not be granted. During the tests the performance and handling of the aircraft are checked with various loads on board. This photograph shows a load consisting of small bags of weighty material being put in place on an aeroplane.





[From Part 19, published 12 July 1938]



You can read more on “Aeroplane Engineering as a Career”, “Pioneers of Large Aircraft” and “Work of the Test Pilot” on this website.








The Certificate of Airworthiness