Semper Lightning: F-35 Flight Control System

The inaugural issue of Code One contained the first of the Semper Viper series of articles, in which Joe Bill Dryden, a senior company test pilot, gave an insider’s view of the F-16. Joe Bill’s passion, insight, and clarity provided a shining light – and a lightning rod, sometimes – for a generation of F-16 pilots.

Why is a series like this needed? The reason is as valid for the F-35 as it was for the F-16: The flight manual doesn’t tell the whole story. It’s not intended to. While there is always debate about what should be in a flight manual, the authors, generally, try to keep the document minimal and austere. (You knew you were in trouble when they named the F-35 manual “Flight Series Data,” right?) If you’re like me, though, you want to know more. You want to know not only how something works but why it works that way. Understanding tradeoffs and design rationale is an important part of learning to use a machine well.

My hope is that these articles, by giving some insight into how the F-35 works and why it was designed that way, will help pilots fly the airplane better, employ it more effectively, and handle emergencies more intuitively. The articles may also help dispel some myths. As Joe Bill put it, “as airplanes become more complex and more capable, the amount of misinformation seems to grow.” With your help, we can keep the mythology in check.

Some of this information may seem academic. That’s partly because I’m a geek and find the technical details interesting (I hope you do, too). But it’s also because design decisions involve tradeoffs, and it’s important to understand them. The F-35 systems, training, and tactics will evolve over the next several decades, and fleet pilots will be intimately involved in that evolution. Anything we can do to increase our technical understanding as users will raise the game for everyone involved in this effort, and will ultimately result in a more effective pilot cadre and weapon system. 

Feedback is critical to making this series a success. With Code One online, we can turn what used to be a lecture (the print version) into a conversation. Let us know what you’re interested in, and we’ll hunt it down. Let us know what you disagree with (be gentle!), and we’ll find the best answer and learn together. You can reach us by using the comments section, or by sending an email to the editor. Stay in touch.

F-35 Flight Control System, Part One

When people think of the “miracle” of flight, they usually think of overcoming gravity. Turns out, that’s the easy part – some things overcome gravity even when we don’t want them to (think race cars, and roofs in tornadoes). The hard part is control. Indeed, control – not lift or propulsion – was the key to the Wright Brothers’ fame and the subject of the years-long patent war that followed. Flight control systems have evolved continuously since the Wrights’ first flight, and the F-35 represents a historic step in that evolution.

In his first article for Code One, Joe Bill did a great job introducing us to fly-by-wire (FBW) control. The F-16 was the first production fighter to use FBW, so Joe Bill had plenty to talk about. As he said, the F-16 was…different.

In this article, we’ll discuss FBW, generally, and focus on some features of the F-35’s control laws (CLAW) from the pilot’s perspective. In the next article, we’ll get into some engineering details and see what’s so innovative – and historically significant – about the F-35’s approach to FBW.

Why Fly-by-Wire?

Forty years ago, when Harry Hillaker and his design team decided to incorporate FBW control in the YF-16, the decision was hardly a slam-dunk. When asked what he considered the riskiest feature of their design, Mr. Hillaker didn’t hesitate: “The fly-by-wire system. If the fly by wire didn’t work, our relaxed static stability wasn’t going to work.” [1] To manage the risk, they had a backup plan to mount the wing further aft, reverting the airplane to a statically stable (albeit draggier and less maneuverable) design that could be flown with a conventional flight control system. 

Today, FBW is so accepted, and so beneficial in terms of reduced weight, survivability, design flexibility, and performance, it’s hard to imagine a modern fighter controlled any other way.

The F-35, in most of its flight envelope, is unstable in pitch and neutrally stable in yaw. What that means is that if there were a nose-up or nose-down disturbance that the stabs didn’t immediately react to counter, the disturbance would grow. RAPIDLY. At normal cruise speeds, the time for an angle of attack (AOA) disturbance to double, if not corrected, would be about a quarter of a second. This instability makes the airplane agile and highly efficient aerodynamically, but it would also make it unflyable were it not for the flight control system – doggedly, eighty times per second – positioning the stabs to keep the nose pointing into the wind. So, as golden-armed as we F-35 pilots are, if we were responsible for positioning the control surfaces ourselves, the airplane would be out of control in seconds.

Static stability isn’t the only thing artificially created in a FBW airplane. The dynamic response – the way the airplane responds to our control inputs – is also created artificially. That response can, in fact, be just about anything we want, since it’s determined by software…not nature.

What? We Don’t Like Nature?

Have you ever known someone who did exactly what you asked? (Okay, me neither, but work with me here.) FBW airplanes are a lot like that guy. Their response is, in a way, too perfect: they do exactly what we tell them. As a result, we have to un-learn some of the compensation we thought was “just part of flying.” 

For example, when we want a snappy roll in a mechanically controlled airplane, we have to overdrive the stick to get the roll going, then apply a check in the opposite direction to stop it. Not so in our computer-controlled machine. The F-35, as most FBW airplanes, sees our lateral input not as a command to move a surface but as a command to provide a roll rate: it overdrives the surfaces to get the roll going, then backs them off to maintain the rate we’ve commanded. When we remove the command, it drives the control surfaces against the roll to bring it to a crisp stop. If we check, as we did with basic airplanes, the airplane obediently performs a quick head-fake in the direction of the check. Most of us experienced that in our first flight in a FBW airplane, but the tendency went away quickly as we learned the new response.  

Another example is turn coordination, which relates to the amount of sideslip we get during rolls and turns. Automatic coordination isn’t unique to FBW: we’ve had aileron-rudder interconnects (ARIs) for years, and even the Wright Flyer had one[2]. But turn coordination in FBW airplanes can be very sophisticated. Generally, the F-35 tries to keep sideslip near zero, but in some cases it intentionally creates adverse or proverse yaw as necessary to control roll and yaw rates. We’ll talk about the use of pedals at high AOA in a later article, but, for general flying around, the best coordination we’ll get is with our feet on the floor.  

The point is: When we move the stick and pedals, FBW gives us what we actually want – or what the control engineers want us to have – while suppressing the extraneous things nature has always tossed in along with it, things we previously had to compensate for or just learn to live with. 

But Wait, There’s More!

FBW does more than just stabilize the airplane and clean up its response. It determines the very nature of the response itself. That response can be programmed to be whatever we want, as a function of the airplane’s configuration, speed, or whether it’s in the air or on the ground. For example, if we make a lateral stick input in CTOL mode, we get a roll rate. But in jetborne mode, we get a bank angle. At high speed, a pitch stick input commands a normal acceleration (“g”); at low speed with the gear up it commands a pitch rate; at low speed with the gear down, it commands an AOA; and in the hover, it commands a rate of climb or descent. 

The ability to tailor the airplane’s response as a function of its configuration and flight regime is the beauty – and potential curse – of FBW. If control engineers get it right – if they define the modes properly, put the transitions in the right places, and give the pilot the right feedback – then control is intuitive. But if they make the various modes too complicated, or the feedback (visual or tactile) isn’t compelling, then modal confusion can set in and bad things can happen. 

Some mode changes occur without our knowing, which is fine as long as we don’t have to change our control strategy. An example is the blend from pitch rate command at low speed to g-command at high speed. This transition is seamless from the pilot’s perspective.

Other changes require us to change our technique, which is okay if we command the changes ourselves and they’re accompanied by a compelling change in symbology. Examples are the transitions from gear-up (UA) to gear-down (PA), and from CTOL to STOVL. 

There are few areas, though, where a mode change is important but not obvious, which is where pilot discipline and training come in. For example, the CV airplane has three different approach modes, easily selected using buttons on the stick and throttle. Two of these modes – APC and DFP[3]  – are autothrottle modes, indicated by a three-letter label on the left side of the HUD. The third mode – manual throttle – is indicated by the absence of a label…arguably not the most compelling indication that you’re responsible for the throttle. This interface will probably evolve; in the meantime, we need to be disciplined and to make doubly sure we’ve got APC engaged before we turn throttle control over to George. 

Another area is STOVL landing. The difference between what the power lever (a.k.a. throttle) does on the ground and what it does in the air is profound. On the ground, it acts like a normal throttle: pulling it full aft commands idle thrust. In air, it commands accel/decel rate: pulling it full aft commands a maximum decel. There’s plenty of redundancy in the weight-on-wheels sensors, but if the airplane ever thought it was still airborne after a vertical landing, and you pulled the throttle full aft, the airplane would go charging backward. This would be “untidy” (as our British friends say), especially on the ship. So we take every STOVL landing to a firm touchdown, and let the airplane itself set the throttle to idle when it determines it’s on the ground.

Protecting Us From Ourselves

The control limiters in the F-35 – love them or hate them – are there to help. They not only make the airplane safer, but also more effective, by allowing us to fly aggressively without worrying about breaking something or losing control. 

But flying the F-35 is not completely carefree. The control engineers had to give us some rope in a few places, since doing otherwise would have compromised capability and possibly even safety. So it’s important for us to understand what’s protected and what isn’t.    

One of the things CLAW does not protect us from, for example, is overspeed. We can exceed Mach and KCAS limits in nearly every configuration (Mode 4 being the exception), though an OVERSPEED caution alerts us as we approach them. 

What about g? We’re mostly protected, but not completely. Interestingly, the protection is least where the maneuvering limits are the lowest: in powered approach (PA) and aerial refueling (AR). The limits in those modes are 3g and 2g, respectively, and there’s nothing to keep us from exceeding them. Why not? Because, while those limits are more than adequate for normal ops, there might be times when we need to exceed them to avoid hitting something – such as the ground, or the tanker – and our CLAW engineers have wisely decided that running into things would probably be worse than busting the g limit. So they let us bust the limit. 

What about high-g maneuvering, up-and-away? For symmetric maneuvers, CLAW’s got our back: As long as we’re not rolling or yawing, we can slam the stick full aft or (ugh) forward, at any speed, at any loading. CLAW will keep g within NzW limits[4].

Rolling and yawing – so-called “asymmetric maneuvering” (maneuvering using lateral stick or pedal inputs) – is another story. If we don’t pull more than 80 percent of the positive NzW limit or push to less than negative 1g, we can roll and yaw to our heart’s content. But if we push or pull more than that, we have to abide by a pilot-observed limit of 25 degree/second. (Stick your hand out in front of you and roll it through 90 degrees while counting to three potatoes. Yup, it’s slow.) I know what you’re thinking: “How do I know when I’m more than 0.8NzW?” You don’t – unless you’re good at mentally dividing the basic flight design gross weight (BFDGW) by your current gross weight and multiplying it by 0.8 times the basic g-limit for the airplane. (If you can do that, continuously, you’re probably in the wrong line of work.) And, “Why 25 deg/sec?” Because that’s the loads folks’ definition of “zero”: if you’re rolling less than 25 deg/sec, they consider that not rolling, so symmetric limits apply.

But, mostly, you’re thinking: “What’s with the pilot-observed limit? Why couldn’t the control engineers just protect us with CLAW?” The reason is that the analysis and the design work to handle every asymmetric input, under every flight condition and loading, would be prohibitive. And if they put the 25 deg/sec limit into CLAW, it would be tactically restrictive and possibly unsafe. So they picked the middle ground of telling us not to roll too much while we’re on the g-limiter. 

So what happens if we make a big roll input at 0.9 NzW? First of all, the CLAW folks haven’t completely abandoned us: As g increases, the roll rate is reduced, and, if we’re commanding more than 50 deg/sec, the airplane unloads to get us back within the 0.8NzW limit. But there’s no guarantee that the unload will be quick enough to prevent an overload.

Does that mean we can break the airplane by pulling and rolling? Not really. The pilot-observed limits were decreed to make sure the airframe delivers its contractually specified life. If we exceed them, the wings won’t fall off, but we might reduce some of that life. The bottom line: If you’re on the g-limiter and want to roll, back off a little, then roll. This will not only keep you within the rules, it will give you a better roll rate in the bargain. If you can’t back off – because, say, you’re trying not to hit the ground, or trying not to get shot (and I don’t mean by your buddy during BFM) – then do what you need to do! The worst thing that will happen is that you’ll trip an OVER G advisory or an overload HRC,[5]  and have to explain your heroic act to the maintenance officer when you return. Presumably, the maneuver will be worth the airframe life you expend.

We Don’t Need No Stinkin’ Limiters

While almost everyone appreciates limiters that prevent overstress, the consensus isn’t as strong when it comes to those that limit control, i.e. the limiters on AOA and body rates (pitch/roll/yaw) intended to keep us from departing controlled flight. We won’t settle that argument here. Like all design issues, limiter design is a tradeoff between competing requirements – in this case, between agility and departure resistance – and opinions will always differ regarding where that line should be drawn. Some pilots will argue (as some have) that we should get the limiters out of the way, or at least open them up, and leave it to the pilot to learn where the cliffs are. The counter-argument is that, assuming we’ve got the limiters in a reasonable place now, opening them up would result in more departures, some of which may cause overstress and some (if they happen at low altitude) loss of the aircraft altogether. 

The F-35 is an inherently unstable airplane, required to handle a wide range of CG. Its control surfaces are sized to meet the requirements of both maneuverability and low observability. As a result, the combinations of body rates, AOAs, CGs, Machs, and weapon bay door positions that define the controllable envelope of the F-35 are extremely complex – and the boundaries of that envelope are reflected, with all that complexity, in CLAW. If the control engineers opened up the limiters and gave us, instead, “rules of thumb” to maintain control – ones that we had a fighting chance of remembering – the rules would most likely be so restrictive that we’d give up more than we gained. Could we evolve to that in the future? Sure, if we decide it’s a positive trade. As the control engineers hate to hear us say, “It’s only software.”

How Does It Do It?

In this article, we talked about what the FBW system does. But we didn’t talk about how it does it, i.e., how it figures out which effectors to move, how much to move them, and how to handle failures. It isn’t magic, but it’s close. To appreciate the historical significance and engineering brilliance of this machine – and, more importantly, to impress your friends – you’ll want to take a peek at what’s going on under the hood. The second article will address how FBW works in the F-35, and why it was designed that way.

Dan Canin is a Lockheed Martin test pilot based at the F-35 Integrated Test Force at NAS Patuxent River, Maryland.

Footnotes

[1] Code One (Vol 6, No 1, April 1991) [return]

[2] The Wright brothers incorporated automatic rudder coordination because they had no choice. Laying prone and controlling the aircraft’s roll with their hips, there was no practical way to control the rudder independently, so they linked the rudder wires to the wing-warp hip cradle. Interestingly, the Wrights deleted the interconnect in their later models, preferring to have direct control of sideslip and to rely on pilot skill for coordination. It was decades before airplanes incorporated both: automatic coordination with roll, with additional yaw command available via the pedals. [return]

[3] We’ll talk more about these in a later article on advanced control laws, but for now: APC is “approach power compensation” mode, in which the throttle is automatically controlled to maintain the desired AOA during approach. In the C-model, engagement of APC also increases the gain on IDLC (integrated direct lift control), which schedules the flaps in response to stick movements to give very high-gain glideslope response. Another approach mode, DFP (delta flight path), currently in the C-model only, changes the pitch axis CLAW from a pitch-rate system to a glideslope-command system. DFP improves glideslope tracking performance and significantly reduces workload during carrier approaches. [return]

[4] What’s NzW? The airframe structural limit isn’t just a function of g – which the pilot can sense – but the actual lift force imposed on the airframe, which is the product of gross weight (W) and g (also known as Nz, the normal acceleration in the z direction). At light weight (low W) we can pull more Nz with the same structural load (Nz*W). That said, there’s still a maximum g the F-35 is allowed (9g for the F-35A, 7g for the B, and 7.5g for the C), and CLAW will let us pull that anytime the weight is less than the Basic Flight Design Gross Weight (BFDGW). Above that weight, the allowable g decreases to keep the total lift – Nz*W – constant. . Fortunately, CLAW figures that out for us. [return]

[5] An OVER G advisory will trip if you exceed the book symmetric or asymmetric maneuvering limits by more than 0.5g. For the purposes of this ICAW, the airplane defines as “asymmetric” any roll rate over 50 deg/sec, so there’s a 25 deg/sec buffer there as well. So if you stick to the flight manual roll rate limit, you should never see this ICAW. What you might trip, though, is an “overload” HRC, which has a much more sophisticated algorithm behind it and will only trip when you’ve exceeded an actual limit on some component of the structure. CLAW should in all cases prevent actual overload to failure, but during rolling maneuvers it may allow one of these indications to trip, requiring a maintenance inspection. [return]