Most of us don’t give aircraft engines much thought. Sometimes, when you’re in the sky, you see the engines hanging under the wing and maybe ponder the sheer improbability of physics and everything holding together at 37,000 feet. Then you go back to your wine and iPad, comfortable that despite everything, it will all work. We take aircraft engines for granted, assuming they will work flawlessly. The manufacturer’s records and reputations assure us. But there is a lot of work put into getting us so comfortable and reassured.
There are several engine manufacturers around the world. Most of them are big names – General Electric, Rolls Royce, and Pratt & Whitney. They’ve been supplying engines to the aircraft manufacturers for years. While there have been hiccups along the way and some aircraft types have been plagued with engine issues, they are usually relatively minor. One thing that characterizes modern jet engines, regardless of manufacturer, is their reliability.
Aircraft engines are meticulously engineered and manufactured. But once built, the engines are put through a relentless testing process. Here, we look at how aircraft engines are tested before production.
Water and ice fired into the test engines
Engines can take in large amounts of water, so water ingestion tests are standard. At Boeing, planes taxi through specially made water troughs. Manufacturers also force a stream of water into running engines. When General Electric tests its GEnx engines, it fires 800 gallons of water per minute into the engine. That water should pass through the engine and be shot out the back without reducing thrust.
It means aircraft engines can handle the fiercest rainstorms and running through pools of water.
“These tests assess the correct functioning of engines and thrust reversers, as well as braking systems when submitted to water thrown up by the wheels in cases of standing water on the runway,” says Boeing told the BBC’s Katia Moskvitch.
Stepping it up a notch, testers will also shoot ice into running engines. These are not dainty little icicles either. Rather, they are big balls of hard compacted ice. It mimics the effects of flying through a hailstorm.
How chickens help deal with a bird strike
We report a lot on bird strikes here at Simple Flying. A bird strike can occur when a feathered friend hits the cockpit window or other aircraft part. Birds can also get ingested into engines. Because engines are now so well engineered and built, bird strikes rarely cause fatalities (except to the bird).
These days, there is one human fatality attributed to bird strikes per one billion flying hours. But bird strikes do damage aircraft, resulting in $1.2 billion worth of repairs annually. And, as the US Airways controlled landing in the Hudson River in 2009 demonstrated, bird strikes can still down a plane.
Engine manufacturers have a low tech but effective way to test an engine’s ability to withstand a bird strike. In the 1950s, de Havilland invented the chicken gun. It’s a large-diameter, compressed-air cannon that fired chickens carcasses at the plane, including into its engines.
The desired outcome is for blades to hold their form after the collision. Since the 1950s, fresh chicken carcasses have been swapped out for frozen carcasses.
Adam Tischler, of Boeing’s Flight Test Communications Division, told the BBC;
“We have used fowl to test aircraft structures. This is not a common test. However, it can be an effective way to evaluate the results of a bird strike on an aircraft.”
Exploding blades and containing debris
One of the toughest tests of engine reliability is simulating what happens when a blade in the engine fan comes loose. That shouldn’t happen, but it has been known to occur. It’s a possible issue if the engine ingests debris.
A jet engine shaft can spin at 3,000 revolutions per minute. If a blade comes off, for whatever reason, it can cause enormous damage to the rest of the engine, and debris can impact the fuselage.
What engine manufacturers try to do is contain the blade within the engine and allow the casing to diffuse the energy. During the testing process, a small explosive will get attached to the base of the blade. They run the engine, detonate the explosive, and ensure that the blade stays within the engine chamber.
Testbeds and testing engines in the sky
These types of tests occur while aircraft remain safely on the ground. But the real test of a jet engine’s reliability is when it flies. Engines need to get certified as safe and fit for purpose. Part of that certification process is taking the engines into the air. For that, the engine manufacturers use modified aircraft known as testbeds.
Retired 747 aircraft sometimes find a second life as testbed aircraft. General Electric has two converted Boeing 747s packed with computers, electronics, and other gear. Rolls Royce also uses two old Boeing 747s as testbed aircraft.
Rolls Royce picked up their latest testbed aircraft in 2019 from Qantas. In a statement at the time, Rolls Royce said;
“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.”
With an awkwardly positioned engine, testbed aircraft might look odd, but they are integral to testing engines and no small investment on the engine manufacturer’s part. Rolls Royce spent US$70 million on buying and modifying that Qantas 747 last year.
Rolls Royce will soon take another retired 747 aircraft off Qantas’ hands. There are rumors General Electric is also looking at buying one or more. Why does Qantas offload so many 747s? The old planes may be aging, but they are obsessively maintained, and the engines remain in tip-top condition.
Going above and beyond
Once manufacturers like Airbus and Boeing are ready to roll out a new plane, they put the engines and the aircraft through their own tests. When getting the A350 XWB ready for final certification, Airbus flew the plane tens of thousands of miles over hundreds of hours. These final flights test all aspects of the aircraft’s performance, not just the engines. But, you know, the engines are kind of important.
The engine manufacturers, the aircraft manufacturers, the airlines, and passengers want reliability. While nothing is perfect, they want to get as close to it as possible. As Airbus says, it is a matter of going above and beyond what is required.