Aircraft engine certification process | Pursuit for 100% perfection

Overview: Aircraft engine certification

Every time an aircraft rolls off the production line, its goes through a series of stringent tests before we see them take to the skies. A '' Type Certificate'' is issued which ensures the aircraft model is compliant with all applicable airworthiness requirements and safety standards and is eligible for mass production. The same applies to ''engines'' as well. 

General Electric, Rolls Royce and Pratt & Whitney are the top names when it comes to engine manufacturing and they've been powering aircrafts for decades. As we sit comfortably in our seats sipping on a glass of wine, we often take the reliability of these engines for granted, given the manufacturer's records and reputation. While that is true, it is to be understood that a lot of work is put into making that happen. Let's take a look.

rolls royce - indoor testing phase

The need for engine testing

Like anywhere else, testing is the last and the most indispensable part before engine certification. Engines have to go through a series of tests, both on-ground and in-flight before the airworthiness authorities give the final go-ahead. The main task of the testing team is to make sure the engine works in accordance with all the parameters set by the engineer. The engine will be tested in all possible scenarios mimicking conditions during taxiing, take-off ,descent, cruise, approach, landing and some other. (As we will soon see)

Water Ingestion

Engines ingest massive amounts of water during their operational life hence water ingestion tests are standard. At Boeing and Airbus, airplanes are made to taxi through special water channels known as "troughs". As the planes speed through these troughs, various engine parameters such as thrust, temperature, engine blade performance etc are evaluated and any deviation from the expected ideal performance is worked upon further.

airbus - water ingestion test

The engines have to weather the fiercest of rains and the hardest of storms. For this very reason, manufacturers spray gallons of water, for instance, The GeNx pumps 800 gallons of water per minute into the engines during test run. Now technically if the engine meets all the design and safety parameters the water is expected to ideally come out without harming the engine frame or its safe operation. Icy conditions can get a little tricky as the FAA mandates that for various different types of ice particles, and we're talking huge balls of ice and not small icicles, the engine will recover as soon as possible without much damage.

rolls royce

A bird for a bird

Remember the crash landing of US AIRWAYS on the Hudson river in 2009 by Captain Sully? It serves as a grim reminder to the fact that even our feathery friends can do a fair amount of damage to jet engines. Bird strikes are common and when they do happen, birds-especially the big ones can bend the blades at the front of the engine causing an inflight engine shutdown or stall. Hence manufacturers have devised a, what is called a ''chicken gun''-that shoots chicken carcasses into the engine, mimicking a bird strike in nature and the tests are evaluated. Ideal outcome of the test is for the blades to remain intact upon impact and post collision.

Pratt & Whitney

Blade-Off procedure

Touted to be one of the most violent tests of all. This simulates an event where an engine blade snaps off the revolving shaft- the shaft could be revolving at 3000 RPM, and turns into shrapnel. The shrapnel turns into flying debris and could potentially tear through the fuselage and the rest of the plane if it is not contained. Hence this procedure aims at containing the shrapnel within the engine chamber and diffusing the energy using the casing.

Inflight Testing

In addition to ground tests, jet engines are also subjected to evaluations while in flight and this puts the ultimate engine reliability to test. For this very purpose, manufacturers use ''testbeds''-which are basically modified aircrafts used for testing. Most of the B747s that are retired find use as testbeds.

rolls royce - b747 platform for engine test

“As a flying testbed, it will get fitted with the latest testing capabilities and, for the first time, will test engines which power both commercial and business aircraft. New systems will obtain better data faster than ever before, and technologies will get tested at higher altitudes and faster speeds” – Rolls Royce (2019)

Summing up

Once the plane manufacturers are ready with their models, they run those models through a series of their own tests to ensure all aspects of flight safety, aircraft and engine performance are met to the highest standards as defined by aviation regulations worldwide. In Airbus’s words,'' It is only a matter of going up and beyond what is required ''- Truly said.

For those of us who are not aviation enthusiasts, the idea that aircrafts sometimes dump fuel just after the take-off or right before they land might sound psychotic and deranged. But it turns out that the practice of “fuel dumping” or “fuel jettisoning” is a fairly well-established procedure used by planes primarily in certain emergency situations to lessen the aircraft’s weight. So why would pilots dump costly fuel onboard, which you as a passenger has just paid for? Did the pilots miscalculate the fuel required to go from A to B? Perhaps not.

Cockpit Panel

Why is such a procedure performed at all?

The most probable rationale is a necessity to attempt an emergency landing soon after take-off. This could be due to an mechanical fault, a bird strike or a medical emergency, and since maximum take-off weight(MTOW) is far greater than maximum landing weight(MLW), the pilot needs to dump fuel. For example, an Airbus A380 carries 250 tonnes of fuel when fully topped up. While the maximum take-off weight of the aircraft is 575 tonnes, the maximum landing weight is 394 tonnes only, whopping 181 tonnes less. Landing with a heavier aircraft than that figure risks buckling the undercarriage, with potentially catastrophic results.

Aeroflot Flight 1492 (5 May 2019) was a scheduled passenger flight from Moscow to Murmansk suffered electrical failure due to lightning strike on climbing out. Captain's injudicious decision to land back immediately without considering landing weight limitations resulted in bounced landing and hard touchdown, causing the landing gear to collapse, fuel to spill out of the wings, and a fire to erupt. The fire incinerated the rear of the aircraft, killing 41 of the 78 souls on board. On 2 October 2019, investigators filed criminal charges against the captain for not following guidelines in aircraft manuals for safe operation.

What to consider ? Overweight landing - Jettisoning - Holding pattern to burn off fuel

Due to steep increase in the cost of fuel, airlines are hesitant in deciding whether to land overweight, burn off fuel, or jettison fuel. Each choice has its own set of factors to consider. Holding to burn off fuel or jettisoning fuel prior to landing will result in increased fuel cost and time-related operational costs. Landing overweight requires an overweight landing inspection with its associated cost. Most operators provide their flight crews with guidelines to enable the pilot to make an intelligent decision to burn off fuel, jettison fuel, or land overweight considering all relevant factors of any given situation.

credits : Devinder Sangha

A pilot will choose to dump fuel only on very rare occasions. Unless there is a medical emergency on board, someone is dying and you don't really have time to fly around and burn fuel. So, that's when you would dump fuel so that you can lose weight quickly. In any which case, for an aircraft to land safely, the MLW must always be lesser than the MTOW.

Not elementary as opening a tap

Ideally, the minimum altitude for a fuel dump is around 6000 feet above ground level. Fuel is ejected at the rate of several tonnes per minute through valves on the trailing edge of the wings, close to the wingtips and away from the engines to avoid combustion. Even though fuel is vaporized, it is still suspended in the atmosphere. The odour can be pronounced, and the fuel will eventually reach the ground.

Because of the relatively small amount of fuel that is jettisoned, the infrequency of use, and the safety issues that may require a fuel jettison, such regulations are not likely to be promulgated. Other factors such as fuel jettison nozzle dispersion characteristics, airplane wake, and other atmospheric conditions can affect the amount of fuel that reaches the ground.  Generally speaking, designated areas are above water bodies or unpopulated regions above land.

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Delta Airlines Flight 89 is an example of fuel dumping that violated established regulations: on 14 January 2020, it dumped more than 10,000 gallons of fuel at a low altitude over a populated area in Los Angeles, causing injuries to 56 people including school children below.

How long is the process

Only long range aircraft such as the B777 or A380 have a fuel dump mechanism. Manufacturers have not been able to provide a specific rate since the dumped fuel is not pumped but delivered by gravity feed so as to be more independent of electrical systems, which might be unavailable in a fuel-dump scenario. As a rule of thumb for the Boeing 747, pilots quote dump rates ranging from a ton per minute, to two tons per minute, to a thumb formula of dump time = (dump weight / 2) + 5 in minutes.

B767 Panel

Many planes are not fitted with the fuel dump feature, as it happens. That’s the case across most narrow-body planes like the B737, A320 and most regional jets. That’s because they meet strict parameters laid out by aviation regulators showing they can still perform critical manoeuvres and hence, doesn't compromise on safety.

On 27th May 2021, Flight crew of an Air India flight (AI105) serviced by a Boeing B777-300ER operating from Delhi to Newark decided to return due to a bat in the cabin. Passengers and Flight attendants noticed the mammal around 20 minutes after take-off. However, landing a Boeing 777-300ER filled with fuel for a 15-hour flight is not simple as it seems. Before landing back safely, the crew had to fly numerous holding patterns for over two hours in the vicinity of Delhi Airport and dump fuel to reach the maximum landing weight permitted.

What Indian regulation says

Directorate General of Civil Aviation (DGCA) has a handful of regulations in place: the flight crew shall request Air Traffic Controller (ATC) for permission. The plane has to be at least 6,000 feet above the highest obstacle along the route. The route to be flown, which, if possible, should be clear of cities and towns, preferably over water and away from areas where thunderstorms have been reported or are expected.

The horizontal boundaries of the area within which other traffic requires appropriate vertical separation extend for 10 NM either side of the track flown by the aircraft which is dumping fuel, from 10 NM ahead, to 50 NM or 15 minutes along track behind it (including turns).

Aircrafts fly through extremes of temperatures and this subjects it to a variety of performance issues. One such phenomenon is the accumulation of ice on aircraft structures which is very common but can turn perilous quickly if not dealt with. Let's take a look at the how and why...

What is aircraft icing and why is it dangerous?

One of the greatest hazards of flying in cold weather is aircraft icing. Aircraft icing refers to coating or deposit of ice on any object of the aircraft, caused by freezing and impingement of liquid hydrometers. It can have a detrimental effect on the aircraft, and it can make it hard for the pilot to fly the plane.

Ice can collect on the surface of the plane and can change the shape of aerofoils and flight control surfaces and hamper the function of the wings, propellers, and control surface as well as canopies and windscreens, pitot tubes, static vents, air intakes, carburetors and radio antennas.

Effects of Icing

Aircraft icing increases weight and drag, decreases lift and can decrease thrust. Ice reduces engine power by blocking air intake. When ice builds up by freezing upon impact or freezing as runoff, it changes the aerodynamics of the surface by modifying the shape and the smoothness of the surface which increases drag and decreases wing lift or propeller thrust. Both a decrease in the lift on the wing due to an altered aerofoil shape and the increase in weight from the ice load will usually result in having to fly at a greater angle of attack to compensate for the lost lift to maintain altitude. This increases fuel consumption and further reduces speed, making a stall more likely to occur, causing the aircraft to lose altitude.

Ice also accumulates on helicopter rotor blades and propellers causing weight and aerodynamic imbalances that are amplified due to their rotation. The first place of an aircraft where ice usually forms first is the thin outside air temperature gauge. The ice generally takes over the wings at the end. Occasionally, a thin coating of ice may form on the aircraft’s windshield. This may occur on landing and take-off.

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Flight instruments may not operate if ice builds up on the static pressure ports of the plane and the pilot tube. The rate of climb, airspeed, and altimeter might be affected. Gyroscope instruments inside the aircraft that are powered by a venture might get affected too when ice builds up on the throat of the venture.

Types of Aircraft De-icing and Anti-icing methods

Pneumatic de-icing boots: The pneumatic boot is usually made of layers of rubber with one or more air chambers between the layers. It is placed on the leading edge of an aircraft's wings and stabilizers. The rapid change in the shape of the boot is designed to break the adhesive force between the ice and the rubber, and allow the ice to be carried away by the air flowing past the wing.

Flight Engineer

Fluid De-icing: These external ground-based systems use a de-icing fluid—typically based on ethylene glycol or isopropyl alcohol to prevent ice forming and to break up accumulated ice on critical surfaces of an aircraft. Fluid is forced through holes in panels on the leading edges of the wings, horizontal stabilizers, fairings, struts, engine inlets, and from a slinger ring on the propeller and the windshield sprayer.

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Bleed Air: Bleed Air systems are used by most large aircraft with jet engines or turboprops. Hot air is "bled" off one or more engines' compressor sections into tubes routed through wings, tail surfaces, and engine inlets. A disadvantage of these systems is that supplying an adequate amount of bleed air can negatively affect the engine performance. Higher-than-normal power settings are often required during cruise or descent, particularly with one or more inoperative engines.

“The Boeing 787 Dreamliner uses electro-thermal ice protection. In this case the heating coils are embedded within the composite wing structure. The system uses half the energy of engine fed bleed-air systems, and reduces drag and noise.” – Boeing

B787 Thermo electric Anti-icing system

So what makes the difference between Anti-Icing and De-Icing?

Anti-icing equipment is turned on before entering icing conditions and is designed to prevent ice from forming in the first place. De-icing equipment is designed to remove ice after it begins to accumulate on the airframe.

Some notable air crashes due to ice accumulation

Air France 447: A scheduled passenger flight, an Airbus A330 from Brazil to France crashed into the Atlantic on 01 June 2009 killing everyone onboard. The aircraft crashed after temporary inconsistencies between the airspeed measurements—likely due to the aircraft's pitot tubes being obstructed by ice crystals—caused the autopilot to disconnect, after which the crew reacted incorrectly and ultimately caused the aircraft to enter an aerodynamic stall, from which it did not recover.

Aero Caribbean Flight 883: Another scheduled flight(ATR-72) from Haiti to Havana (2010), encountered severe icing conditions at 20,000 ft which were not handled properly, leading to the crash.

The Covid-19 pandemic has hit a severe blow to the economy worldwide and none has been hit as bad as the aviation industry. Not one to be put down, aircraft manufacturers have been looking for ways to beat around it and one such aerospace giant, Airbus, is already looking towards a new plane to help drive its recovery and get a leg up on rival Boeing.

What is the A321 XLR?

The newest offering of the A320 family, the A321XLR offers a vast 4700 NM of range, far surpassing any current narrowbodies. The longest range Boeing B737 MAX offers is 3850 NM while the A321LR has a range of 4000 NM. The XLR stands for Extra Long Range and the aircraft can more than handle crossing the North Atlantic, opening up the possibility of airlines using it on routes that have been dominated by the wide-bodied aircraft syndicate up to now.

What sets it apart..

INCREDIBLE RANGE : As the very name suggests, this marvel of technology pushes the range to the highest of any narrow body present today- 8,700 km (4, 700 NM). A typical A321 NEO has a range of just under 6,000Kms while a B737-8 reaches out to about 6,570Kms.

Now unlike the A350 XWB(Extra Wide Body), the A321 XLR isn't exactly fresh-off-the-menu design. Airbus has been constantly upgrading the A320 family and this is a result of one such upgrade only. It has already bagged a staggering 450 orders from over 20 airlines and is scheduled to start deliveries by 2023.

Airbus

This aircraft will provide airlines with a range of up to 4,700nm and a 30% lower fuel burn and CO2 emissions per seat compared with previous-generation aircrafts. The aircraft is positioned at what is known as the "middle of the market" in the aviation industry -- the gap between single-aisle narrow-body aircrafts and twin-aisle wide-body aircrafts.

The Rear Centre Tank- the XLR performance masterpiece

The RCT, which is unique to the new long-range A321XLR, is a permanently installed high-capacity fuel tank that makes maximum volumetric use of the aircraft’s lower fuselage.

Integrated in fuselage sections 15 and 17 and located behind the main landing gear bay ,it holds up to 13,100 litres, which is more fuel than several Additional Centre Tanks (ACTs) combined could previously hold in the A321 aircraft Family.

Airbus

In parallel to this, the structural assembly of section 15 started in mid April with the integration of the shells from the supplier RUAG, the Centre Wing Box coming from Airbus Nantes, and the keel beam and Rear Flange Module supplied by Premium AEROTEC.

A setback

Well Boeing has expressed safety concerns with the European Union Aviation Safety Agency (EASA) over the A321XLR. This is in response to a consultation paper by the regulator about Airbus’s plans to install insulation panels on the floor of the A321XLR. Considering B737 Max's never ending issues, Boeing’s feedback is definitely ironic and not with the purest of intentions, but still fair, since ultimately all safety concerns should be addressed.

In a filing, Boeing’s director of global regulatory strategy stated that: “Fuel tanks integral to the airframe structure inherently provide less redundancy than structurally separate fuel tanks.”

The EASA has accorded the Boeing’s concerns around the “structural crashworthiness” of the tanks, and the risks of fire due to heat transfer from an external threat. Airbus now needs to prove through tests that these fuel tanks are as safe as previous designs.

Airbus

What it could mean for Indian Aviation?

Indigo, the largest Indian carrier broke news last October when it announced the order of 300 A320 neo which also contained the new A321XLR- the longest range widebody ever proposed. The XLR’s range gives airlines the opportunity to start routes that weren’t possible with widebodies. The range of the A321XLR means that long haul routes in Europe, Asia and Africa are suddenly now a possibility from Delhi-Indigo’s base hub. The list goes on to include London, Paris, Tokyo and Addis Ababa as well.

Airbus - a320neo delivery to indigo

So far there are no low-cost alternatives available for long-haul and the addition of the A321XLR could just change that, and possibly capture a whole new market.

The upshot

With the pandemic having decimated air traffic, airlines are likely to have even more need for such aircraft as they rebuild their route networks. An A321 XLR "costs much less to buy and service" as well as fly than a wide-body aircraft.

"Pilot training -- an important element of costs -- can be mutualised between long-haul operations and those for short- and medium-haul flights. With the development of the A321XLR, Boeing faces a very serious mid-market challenge. This aircraft is scheduled to enter service in 2023.

Overview

A hydrogen powered aircraft uses hydrogen rather than fossil fuels such as aviation fuel. Hydrogen can either be burned in a jet engine or can be used to power a fuel cell which generates electricity. Liquid hydrogen has about four times the volume for the same amount of energy of kerosene requiring need of larger fuselage length and diameter for storage. Increased surface area lowers performance significantly due to increased friction and drag. However, hydrogen is about one-third of the weight of kerosene jet-fuel for the same amount of energy. Therefore the performance is a trade-off of the larger wetted area and lower fuel weight.

Artist’s concept

History and Present

You will be surprised to know that as early as in February 1957, a Martin B-57B flew on hydrogen for 20 min for one of its two Wright J65 engines. On 15 April 1988, the Tu-155 first flew as the first hydrogen-powered experimental aircraft. More recent progress have been made by Austria's Diamond Aircraft Industries to successfully test-fly it's flagship 2-seater, DA20 with hydrogen powered fuel cells. Due to present-day impending horror of global warming, mega-manufacturers like Airbus and Boeing have been forced to look into ways to decarbonise aviation industry. In 2020, Airbus announced plans to develop three different hydrogen-fuelled concepts, named ZEROe, with the aim of developing zero-emission aircraft powered using hydrogen gas turbine rather than hydrogen fuel cells.

airbus

Are other stakeholders making aviation industry a scapegoat ?

Before the pandemic grounded most flights, commercial aviation accounted for about 2.5% of global emissions of carbon dioxide. It sounds like a small proportion of the whole, but it is more than those of Germany (2.2%), and this is not the whole story. Carbon dioxide accounts for about half of aviation's contribution to what is known as its effective radiative forcing – that is, its total contribution to the factors that actually drive a rise in global average temperature. Contrails – water vapour trails from aircraft – are aviation's largest other factor.

Are hydrogen powered engines completely emission-free ?

There is the question of whether hydrogen can be produced at scale and at a competitive price without itself having a large carbon footprint. The great majority of hydrogen used in industry today is created using fossil fuel methane, releasing carbon dioxide as a waste product. Hydrogen can be produced from water through a process called electrolysis, driven by renewable power, but this process is currently expensive and requires large amounts of energy. Only about 1% of hydrogen is produced this way at present.

Will there be enough hydrogen for all ?

Investment in electrolysers – the “clean” technology used to separate hydrogen and oxygen atoms in water – is booming worldwide. As a result, green hydrogen production capacity could achieve a 50-fold increase in the next six years, according to some estimates. This means green hydrogen could be on track to supply up to 25% of the world’s energy needs by 2050.

irena

And this rapid and cost-effective scale-up could not be more timely: drastic solutions are now urgently required if the world is to meet the 1.5-degree Celsius target of the Paris Agreement.

Sanctioned projects at a glance

The International Energy Agency (IEA)’s Hydrogen Projects Database counts nearly 320 new green hydrogen production demonstration projects worldwide. This amounts to a total of about 200 MW of added electrolyser capacity. And new projects are being added on almost a weekly basis.

At present, Europe has less than 1 GW/year of electrolyser installed capacity. However, the European Commission recently announced longer-term plans to install at least 40 GW of electrolyser capacity or up to 10 million megatons of green hydrogen by 2030. As part of these plans, larger electrolysers – with up to 100 MW capacity as opposed to the current 20 MW capacity – are expected to be built by 2024 and installed next to demand centres.

irena

Australia has one of the world's highest volumes of green hydrogen production capacity, including about 30 GW of projects in the pipeline. In Asia, the region’s electrolyser capacity could reach +10 GW over the coming decade, driven by demand from Japan, South Korea and China. Across the pond, the USA is also starting to catch up with plans to develop green hydrogen mega-projects in California, Texas and Utah.

According to a report released by the International Renewable Energy Agency (IRENA), green hydrogen production costs have already begun to fall largely due to a decline in renewable energy costs and further cost savings in electrolysis facilities.

Getting green hydrogen to airports: how will it work ?

Green hydrogen is an energy pathway that forms a critical part of strategy to lead the decarbonisation of the aviation industry. This means architecting the future green hydrogen ecosystem for aviation will need to start now in preparation of an entry-into-service of hydrogen aircraft in near future. According to Airbus, it could look something like this.

airbus

Bottom-line

For now, one thing remains almost certain: hydrogen and E fuels are likely to continue to be substantially more expensive than conventional jet fuel for years or decades to come, limiting their role in greening aviation – unless the other costs of aviation come to be weighed differently.

Overview

A prototype is an early sample or model of an aircraft built to test a concept or process. It is generally used to evaluate a new design to enhance precision by system analysts and users and in a variety of contexts, including semantics, design, electronics, and software programming.

Why are prototypes made ?

When an aircraft program is launched, the manufacturer must manufacture several prototypes in order to test various systems and ensure safe and reliable operation for the following models that enter commercial service. These prototypes undergo rigorous testing and are put through all sorts of challenging conditions, including extreme heat and cold.

"The plane has been dragged, dropped, soaked, forced to hover, shudder and flutter"-Said Boeing of its 747-8 testing.

Now that begs the question-''What happens to the prototype once the testing is done ? ''

airbus

The experimentation continues...

The answer to what happens to the aircraft is that it depends on a lot of factors. For instance:

The B787 program did write off the airframes for the first 6 aircraft due to the extensive number of modifications that were required to those airframes. They went to museums of strategic importance to either the manufacturer or the kick-off customer and their country.

Some of these aircraft end up going into lifecycle fatigue testing or are maintained in the ownership of the manufacturer for follow-on testing that would be for upgrades or newer subsequent models.

General Elecric

If the program is a brand new model or significant technological change to an existing model, it has to go through stability and control testing and flutter testing. That literally beats the airframe to hell. It will most likely experience an over-G loading in these tests, and therefore, cannot and should not be used to carry passengers. There would be a potential for cargo in these cases, but then there would be added maintenance costs and hence many wouldn't want such a ''beat'' aircraft.

Musee Aeroscopia

It's a continuous process

While an aircraft program might achieve all the necessary certification, and mass production might commence, the prototype units may just continue as test aircraft. Many early models of well-known commercial aircraft became what are known as ‘testbeds,’ – which is defined as: “A vehicle (such as an airplane) used for testing new equipment (such as engines or weapons systems).”

Thus, the teams at companies like Airbus and Boeing use their prototypes to continue the aircraft development process, looking for ways to improve systems further. The testbed may even be instrumental in developing the next family of aircraft – the first B747 was actually a testbed for developing the Boeing 777 engine program.

Musee Aeroscopia

Post testbed phase

Once the testbed phase is finished, the manufacturer will try and find a new home for the aircraft which could typically be a museum – perhaps one that is aviation-centric. For instance, Seattle's Museum Of Flight would be a great place to see the first Boeing 747 and B737 ever built. Heading over to Musée Aeroscopia in Toulouse, one can witness the gigantic A380 (the 2nd one built) and A320 (first built) in all its glory.

airbus

Bottom-line

Prototype aircraft are generally of less value than the production counterparts, since they are heavier and may not have the normal design configuration. Particularly new aircraft (and its derivatives) tend to go to customers as testing becomes more refined and the cause of less design changes. However, not every aircraft can be sold if the aircraft is too unattractive and end up in a museum and hence many necessitate scrapping.