As a liberal arts type, I’ve never been able to get my brain around this issue. It seems to me that water inhibits and/or stops combustion. Why is it that jet and propeller engines on airplanes function in the rain? Does the amount of rain matter? Does the altitude of the plane matter?
Since I’m in an airplane a couple of times a week, I have a personal interest in this issue.
Prop (piston engine)–your car works in the rain; it’s the same type of engine. It just doesn’t inhale much (if any) water.
Jet–the amount of water we’re talking about isn’t enough to affect a big jet.
And anything that you’d fly while it’s raining usually stays above the weather except on takeoff and landing.
Back in the days of piston engines for large aircraft and fighters, some aircraft had water injection. Water (being uncompressable) would be injected into the engine to increase power. I believe (but I’m not sure) that the Harrier aircraft has water injection for the same purpose.
I’ve also heard that the old F-105 Thunderchief had water injection. I knew a guy who was in Vietnam. He said that once an F-105’s water injection tank had been mistakenly filled with JP-4. When the pilot turned on the “water” injection, the engine blew up; but the pilot escaped. I don’t have any way of verifying this story though, and it could have been a joke or an Urban Legend.
Are you sure? I’ve heard water injection did not increase compression on its own; it was used to allow higher boost. The water cooled the cylinder and air-fuel mixture when it turned to steam. This slowed combustion, which worked to prevent detonation.
Admittedly, I’ve only worked on engines that stay near the ground, but it seems that a supercharged fighter engine would have excess supercharger volume at low altitudes to allow high altitude operation. In any case, the simple way to increase compression would be to increase boost.
I have seen a program on the construction of the Boeing 777 and it’s engines. They showed the testing of the engines and the many trials they go through. One engine test was shooting water like it was coming out of a fire hose directly into the engine, no problems. Other tests were equally harsh shooting dead birds into them,
planting an explosive charge to blow off compressor blades, the engine had to keep running during all of these tests. I was very impressed with this one and find it hard to believe, that in development they actually run an engine at 100% power for 10,000 hours! Did I mishear? It seems incredible, if true these jet engines are about the toughest things around.
cornflakes, I don’t know all of the physics of it. Just going off of what I was told years ago. Heck, I don’t even know if piston airliners were still in use when I was born! The closest I got was when my mom bought me a DC-3 ride when I was a kid. No injection on it.
I’m a jet engine design engineer, so maybe I can help:
Water content in air does reduce the efficiency of combustion in any type of engine. Some of the fuel energy that would otherwise be used to push a piston or blow a turbine is used to evaporate the water and separate the hydrogen from the oxygen. If enough water gets in, there’s not enough to keep the engine running.
Piston engine intakes, and some turboprop ones, are designed to keep rain out by providing some corner that intake air has to flow around. Some turbofans are designed to allow water (or other solids) that passes through the big fan up front to go around the engine core entirely.
In a jet, water is vaporized before it reaches the combustion chamber, due to the heat of compression of the air. By the time the air gets there, the air is already at about 800F or so. Before liquefying, the impact of the sharp little compressor blades on rain droplets can cause some erosion damage and efficiency loss, though.
Water injection was used in some of the late big piston engines and some of the early jets for increased power during takeoff, but the water didn’t provide the power directly (“Cornflakes” is right). It served to keep the cylinder heads, or combustor cans, cool so that more fuel could be used without burning the engine up. The increased fuel flow provided the increased power. But the water system had its own weight, using some of the increased power itself, and the higher stresses reduced part life significantly. Modern jet engines produce enough thrust, and have high enough compression ratios, that there’s no need for it anymore. Pity - I’d have loved to see and hear a Boeing 707 “water wagon” take off sometime.
Incidentally, “excessive” turbocharger capacity at low altitudes is controlled by choking down the flow with the wastegate. You still can’t overtemp the engine.
“Icerigger” is right about the FAA certification tests that new engines go through (and is it ever fun). There are prescribed ingestion tests for rain, hail, ice slabs, single large birds, small numbers of medium-sized birds, and flocks of small birds. The major manufacturers have permanent ground-test facilities with custom-built cannons for just this reason. An engine also has to show that a fan blade can break off without penetrating the casing, causing a fire, or tearing the engine off the mounts due to imbalance loads (compressor blade containment can be shown analytically).
Engines also get a fair amount of ground testing at a range of power settings to identify any durability problems. Sometimes we’ll identify a max-vibration point and just sit on it for a long time. Others are simple flight simulations. On military programs, we’ll normally have slave engines simply running all day long on Accelerated Mission Testing cycles, in an attempt to find problems before they can happen in the field. Commercial engines will normally operate so many hours per day in service that factory testing can’t keep up, however.
A commercial engine life of 10,000 hours between overhauls isn’t unusual. Some fatigue-limited parts, like turbine blades, will have to be replaced periodically, but most of the engine can easily run forever. A similar life on a fighter engine would mean that there was capacity to advance the max power setting - it’s worth it to trade life for thrust on those.
Thanks for letting me brag.
I agree fully with cornflakes and Elvis. They said what I was going to post. (D’oh!)
That said, I believe there have been incidents of jet engine flameout in the past in heavy rain. Perhaps not with modern high-bypass fans.
There was a (707? 727?) a number of years ago that had all engines flame out after flying into the ash plume of a volcano. The pilots managed a relight of some or all of the engines and landed without incident, although I believe there was severe damage to the aircraft.
The US metereorological service use piston engined aircraft to fly into hurricanes . The amount of water in the air would cause jet aircraft problems in such conditions.
Yep. The one that first comes to mind is a Southern Airlines DC-9 that had a double flameout in a hailstorm in the '70’s. The crew barely managed to set down on a road. The flameouts may have been due more to mechanical damage than combustor quenching.
It’s also possible with a high-bypass turbofan, even though the fan makes a pretty effective centrifuge. Rain that hits the center of the fan is likely to get ingested in most designs, but not enough to quench the combustor.
I don’t know of any specific instances of a flameout in service due to rain alone, but it’s possible. Perhaps an early fighter design?
You may be thinking of a KLM 747 that hit an ash cloud in southern Alaska about 3 years ago. They barely managed to limp into Anchorage with all 4 engines destined for the overhaul shop. That too was due to mechanical damage to the compressor, not quenching. That fine, gritty ash gets over everything, including inside the turbine blade cooling passages.
Air routes over Japan (which is congested) have to be shuffled whenever they have an eruption, too.
I believe the planes that the National Oceanographic and Atmospheric Administration, which took over this duty from the US Navy, are a combination of P-3 Orions and C-130 Hercules. Both use Allison T-56 turboprops.
Actually, I think the incident you are thinking of was about 10 years ago when a 747 flew into a plume somewhere up north, I think Alaska. All four engines flamed-out. The pilot restarted I think 3 or 4 of them after loosing some 20000ft. The ash coated the insides of the engines, it was miraculous they fired up again. The landed in Anchorage I think without further incident but millions of dollars damage to the plane.
A snip from: http://www.nw.faa.gov/releases/volbook.htm
Talk about having to change you shorts 
Eric
Sorry for the hijack, but I found a better account here:
http://www.geo.mtu.edu/department/classes/ge404/gcmayber/historic.html
Redoubt volcano, near Anchorage Alaska, began erupting on December 14, 1989. On the following day, a 747-400 airplane powered by GE CF6-80C2 engines entered an ash cloud at 25,000 ft. and experienced flameouts on all four engines.
During descent to 25,000 ft., the airplane entered a thin layer of altostratus clouds when it suddenly became very dark outside. The crew also saw lighted particles (St. Elmo’s fire) pass over the cockpit windshields. At the same time, brownish dust with a sulfurous smell entered the cockpit. The Captain commanded the Pilot flying to start climbing to attempt to get out of the volcanic ash. One minute into the high-power climb, all four engines flamed out. Due to the volcanic ash and dust in the cockpit. The crew donned oxygen masks.
The Pilot Flying noticed the airspeed descending, initially at a normal rate (given the airplane’s altitude) but suddenly very fast. All airspeed indications were then lost due to volcanic dust contamination in the pitot system. At the same time, there was a stall warning and the stick shaker was activated with no signs of buffeting. The Pilot Flying rather firmly put the nose of the aircraft down to avoid a stall and initiated a turn to the left in a further attempt to get out of the volcanic ash.
The crew noticed a “Cargo Fire Forward” warning and deduced that the fire warning was caused by the volcanic ash, so no further action was taken.
As the engine spooled down, the generators tripped off and all instrument were lost except for instruments powered by the batteries.
During the time the engines were inoperative, the cabin pressure remained within limits and no passenger oxygen masks deployed. The crew elected not to deploy the masks because the passenger-oxygen-mask system would have been contaminated by volcanic dust in the cabin air.
An emergency was declared when the airplane passed through approximately 17,000 ft. The crew stated that total of seven or eight restart attempts were made before engines 1 and 2 finally restarted at approximately 17,200 ft. Initially, the crew maintained 13,000 ft. with engine 1 and 2 restarted, and, after several more attempts, engines 3 and 4 also restarted.
After passing abeam and east of Anchorage at 11,000 ft, the airplane was given radar vectors for a wide right-hand pattern to runway 06 and further descend to 2,000 ft. The Captain had the runway continuosly in sight during the approach; however, vision throught the windshields was impaired due to “sandblasting” from the volcanic ash in such a way that the Captain and the First Officer were only able to look forward with their heads positioned well to the side. Finally the airplane did land safely, but approximately 80 million dollars was spent to restore the plane, which included replacing four engines. The in-depth account of this incident helped researchers devise a procedure of what a crew should do when they encounter an ash cloud.
Man, that flight would have had a seriously high pucker factor.
I’ve got a tape of the Sioux City DC-10 crash, along with a tape of a lecture Capt. Haynes gave about the whole situation. The pucker factor in that one would have had me pulling buttons off the seat.
>Are you sure? I’ve heard water injection did not increase >compression on its own; it was used to allow higher boost. >The water cooled the cylinder and air-fuel mixture when it >turned to steam. This slowed combustion, which worked to >prevent detonation.<
You guys are both right: the water was used to provide better detonation margins, as it cooled the cylinder.
For those of you not familiar with piston engines, detonation is a pre-ignition caused by octane ratings that are too low or cylinders that are too hot. The explosive ignition occurs too early in the piston’s up-stroke, causing severe damage to the engine. You know it as “knock” in an automobile, but the water cooling of an auto engine keeps it under control. In an air-cooled aircraft engine it’s very dangerous. My partner had an engine destroyed by it in 1998 in a Mooney 201.
But I digress: providing better detonation margins at high power outputs (above 75% power) was the goal.
As a corollary to this thread, many light plane pilots have argued that they see a 5-knot decrease in airspeed in rain. The question is: is it due to lower engine power or the weight/resistance of the rain on the airfoil. Inquiring minds want to know . . .
Think of a turbine engine as a glorified furnace.
As long as there is a flame and you keep pumping fuel in, the engine keeps running.
When you start the furnace, you have a pilot light (ignition), then introduce fuel (natural gas) then the fuel ignites and the flame stays on as long as you provide fuel.
Instead of spark plugs as such, a turbine (at least Pratt and Whitney with which I am intimately familiar) engine has igniters. These provide a “spark” to ignite the fuel at the beginning of a start cycle. If flying in rain, snow, icing conditions, etc., we select ignition to “manual” (on) to provide continuous “spark” in case of ingestion of rain or ice into the combustion chamber (which is a very unlikely event anyway).
The igniters have a limited service life in the order of several hundred hours. They look like a cross between an automotive spark plug and the cigarette lighter in your car.
Regarding volcanic ash, added hazards are it is extremely abrasive (turbine engine components may rotate at up to 35 000 rpm)and I believe also corrosive.
“You screw up just this much and you’ll find yourself flying a cargo plane full of rubber dogshit outta Hong Kong!”
Interesting question. Need more data: Do you actually see an RPM drop for the same throttle setting when you fly into rain? Or do you find you have to apply a little nose-up pitch trim instead? Or both?