I am not a pilot. My understanding of flying large multi-engine passenger jets is that pilots need (or have historically needed) to exercise caution when advancing the throttles to begin their takeoff roll; the issue, supposedly, is that the engine(s) on one side of the aircraft may achieve a high thrust before the engine(s) on the other side, creating a potentially dangerous yaw situation.
First of all, is that true?
Second, if it’s true, does FADEC reduce/eliminate that issue? That is, does FADEC automatically manage thrust ramp rates and compare across all of the engines to assure no large thrust disparity develops during the runup to takeoff power? In other words, does FADEC allow the pilot to firewall the throttle levers and not worry too much about what’s happening after that?
This is correct, or at least was in the past. If the throttles are rammed forward with the turbines at low rpm, in some cases they may spool up at different rates. This can cause unexpected yaw. In one case in 2007 a 737 ran off the runway due to this: https://www.atsb.gov.au/publications/investigation_reports/2007/aair/ao-2007-023/
However all commercial twin-engined jets are required to be capable of withstanding an engine failure yet safely taking off on one engine at max gross weight. So even in that extreme case the plane can be controlled.
But if the thrust asymmetry happened at very low speed (before aero-surface control was effective), it’s possible on a slippery runway the tires might not keep it straight.
I don’t know if the latest jets have safeguards against this. It’s not just FADEC, but some type of differential sensing of thrust (probably left vs right Engine Pressure Ratio) would be needed. Then if that system malfunctioned it might spuriously reduce thrust.
On turboprops engine output is usually measured by torque. Hypothetically you could have similar left vs right auto-matching but what if the torque sensor was erroneous? Automatic systems can introduce additional failure modes that must be protected against.
The main benefit of FADEC is it protects the engines themselves from mis-handling by the pilot, and reduces need to monitor certain engine parameters. With some propellor planes, FADEC permits having a single thrust lever instead of separate levers for throttle, prop pitch (and on a piston plane) fuel mixture.
I think some fly-by-wire aircraft (which is different than FADEC) have some automatic assistance for thrust asymmetry in flight. E.g, if an engine fails it might help auto-trim the aircraft to reduce yaw. I’m not sure if that is intended or usable for takeoff.
I fly a fairly cutting edge business jet, and our FADEC is very sophisticated.
Guarding against an asymmetric yaw was never much on my mind. Rather, FADEC is a great safeguard against over-temping the engines. When I was an airline pilot there were two little red lights just above the engine temp gauges which we called “resume lights”. That is, if those lights came on, you were sending out resumes for a new job because your mishandling just cooked the engines.
In my current jet FADEC sets takeoff thrust for the given conditions. Whatever the temperature, elevation, etc, putting the thrust levers to the forward stop gives you takeoff power. That’s very nice compared to other planes I’ve flown.
It is theoretically still possible to over-temp an engine, but you’d really have to work at it.
I’m flying an A320 these days. FADEC does lots of things, but guarding against assymetric thrust during take-off is not one of them.
It is generally the initial thrust increase that can happen at different rates. The procedure then is to initially advance the thrust levers for an intermediate thrust setting, pause, and then go to take-off thrust.
How do you cook (or avoid cooking) a gas turbine engine? From what I know of them, they run extremely lean (as opposed to gasoline spark engines, which run at/near stoichiometric, and diesel engines, which operate at ~20-80% of stoichiometric), and that extremely lean operation keeps the temps at the turbine blades acceptably low. The only think I can think of (again, not a pilot) is that slamming the thrust lever forward too rapidly results in a high fuel flow rate before the RPM is high enough to provide enough air flow to keep things comfortably lean and cool. Is that about right, or are there other things a pre-FADEC pilot could do to keep engine temps within reason?
Yes, in older jets and turbo props rapid throttle motion is all it takes. Even not so rapid if they’re already at high thrust. A first officer at my airline hit the resume lights during a go-around. The normal pilot reflex is to simply push everything forward, but that can get you in trouble without FADEC to manage things.
As for running lean, note that there is no mixture control on turbine engines.
Turbine temperature is proportional to fuel flow. The more fuel you put in the more thrust you get but also the hotter everything gets. It’s very easy to over temp a non FADEC engine just by pushing the thrust levers forward beyond max thrust.
You can also get an over temp during start. Another way you can get it is if the engines have a high load on them, E.g. running a generator, hydraulic pump and pneumatic systems.
I’d have to dig out some manuals to answer that question accurately. Will see what I can do later tonight. Or better yet, one of our other pilots may post the answer!
Crap, I was really hoping somebody smarter than me would have answered this by now. I know I’m going to mangle it, but here goes…
The basic purpose and design of a jet engine obviates the need for pilot-commanded mixture controls, such as we are used to seeing in piston aircraft (ie, the red knob / lever).
Jet engines compress air. They’re really good at that, it’s their main goal in life. They compress the hell out of air BEFORE it gets mixed with fuel and then burned. And they do this very well more or less regardless of altitude. OK, not regardless to infinity. But jets are able to work well - indeed, even more efficiently - at high altitudes because of their great ability to compress incoming air. The fuel / air mixture is mostly assured by the very nature of the engine design.
As to how it accomplishes this? You get into diffusers, low and high pressure compressors, inlet guide vanes, etc., and I’ll be even more completely out of my depth if I continue this sentence for very much longer. But circling back to the OP, FADEC oversees the details of this stuff inside the engine, and provides the various protections we’ve already discussed.
This all allows me, the dumb pilot who knows far less about the actual oily parts of the engines than you might imagine, to simply shove the thrust levers forward and get thrust approximating what I desired. I am then free to attend to the truly important parts of my job, namely explaining to my rich clients why we mistakenly got them Fiji water instead of the requested Smart Water. And no, I’m not making that part up.
Now please, someone with better knowledge of engine theory explain what I screwed up.
(Note, the following is a guess based totally on many hours of watching this guy on YouTube taking apart jet engines.)
Besides lean/rich, there are two other fairly useful adjustments that can be made in many jet engines: variable guide vanes at the inlet can be adjusted and variable stator vanes in the compression stage can be adjusted.
The former is necessary because compressor blades are little airplane wings that can stall just like an airplane wing, so adjustments to inlet vanes are needed in order to change the angle of attack of the incoming air stream to the compressor blades, so you don’t get a compressor stall on startup and at low speeds.
Likewise, the adjustments of the angle of the stator vanes affects the aerodynamics of the compressor. These both are mostly needed to allow the engine to run at lower speeds that are outside of the normal optimal design speed.
Another option designers have is to bleed air from the compressor at various points.
Presumably these things, along with fuel, are all orchestrated automatically as the pilot asks for power in conditions of changing velocity and density.