Homework, part II

By lex, on June 7th, 2006

Long, repetitive and boring, but the work’s done and maybe one reader may enjoy it.

Or not.

Now give me my “A.”

Abstract: Over the course of the first decade of its lifecycle, the FA-18 Hornet aircraft evolved from a troubled acquisition program designed to fulfill a limited, albeit crucial role on a few aircraft carrier decks into the most successful aircraft program in US Navy history. It did so not by exceeding expectations in any one area, but by being “good enough” in every area, by using modularity in design and by answering cultural and political issues effectively.


Introduction: After the Vietnam War, the US Air Force questioned its philosophical reliance on very high technology, very expensive fighter programs. In order to fill the level-of-effort fighter and ground attack niches for a Cold War battle over the European Fulda Gap, requests for proposal went out to industry in solicitation of concepts for a lightweight fighter program. General Dynamics eventually won the bid with the F-16, while Northrop’s YF-17, the loser in the fighter fly-off, presented an interesting acquisition opportunity for the US Navy. The YF-17′s dual engines offered redundancy – equating to operational survivability – for a Navy operating in the blue water environment, while its ability to carry medium range air to air missiles (in the form of the AIM-7 Sparrow) offered it an adequate offensive and defensive counter air capability. Another advantage was the aircraft’s physical size: The F-14 Tomcat, with its greater endurance, more complex and ostensibly more capable weapons system, was too large to operate from the flight decks of the USS Midway and USS Coral Sea. A new fighter would be required to defend these ships, while all Navy carriers needed a replacement for the light attack A-7 Corsair II.


Design: Nearly every fighter aircraft is built upon the bones of its immediate predecessor. With the lessons of Vietnam immediately in mind during the mid-1970′s, system acquisition professionals were acutely aware of the limitations of the F-4 Phantom in the overland environment, and especially of its relative disadvantages (as compared to lightweight Soviet designs like the MiG-21 Fishbed and MiG-19 Farmer) during close-in maneuvering flight. A new fighter would need a true, look-down, shoot-down radar and the ability to win in the proverbial “knife fight in a phone booth.” But the FA-18 was not only to be a fighter. It was also to replace the venerable A-7 Corsair, a light attack, day, visual bomber. While the A-7 was a good bomber in Vietnam, it had operational disadvantages: As a single engine jet, every motor chug was an emergency, and flameouts almost inevitably resulted in the loss of the aircraft. Neither was it fast, especially with a full bomb load. In combat, the A-7 was too vulnerable to ground fires in the emergent threat environment of increasingly capable surface-to-air missiles and radar guided anti-aircraft artillery.

Design trade-offs are resident in every aircraft design, but an aircraft designed from the ground up to be both a fighter and an attack platform has a particularly high bar to vault: In the lightweight fighter design, premiums are placed on maneuverability and thrust to weight ratios, while the often antithetical attributes of stability and endurance are valued in bombers.

Initially designed to fill a low-end niche, the FA-18 became increasingly important when the A-12 Avenger program ran into serious weight and cost issues, ultimately resulting in program cancellation. Vietnam era A-6 Intruders were expensive to operate and maintain, while rapidly reaching the end of their service lives. Eventually, the A-6 program went into senescence, with newer models of the FA-18 taking their place. As a program in execution as the F-14 approached the end of its useful service life, the FA-18E/F once again offered “good enough” value at an acceptable cost and risk. By adhering to a few emergently apparent DoD acquisition heuristics, the FA-18 practically stumbled into naval aviation dominance by being the “last program standing.” Also, when everyone else (A-12, F-14D, e.g.) is trying to get money to support their program, it’s good to be a program that has money in execution  a program, in other words, that is delivering “shadows on the ramp” rather than PowerPoint slides.

Architecting  Cultural and Political: As mentioned earlier, every aircraft design is a series of trade-offs: We might like a fighter with exceptional endurance to give us operational flexibility in the maritime environment, but fighters with large fuel fractions require very large and thirsty engines and tend to be less maneuverable, especially at higher gross weights. Aircraft designed for high transonic and supersonic flight typically require delta or modified delta wing designs, but such designs tend to be very heavily wing loaded, resulting in high bleed rates a – disadvantage in a turning fight. And while these are tradeoffs in a simple fighter design, ground attack aircraft place a premium on static stability rather than dynamic agility for accurate ordnance delivery. Finally, an aircraft designed for operations aboard an aircraft carrier values a lower approach speed, while swept wing jets typically require very high nose attitudes at optimum angle-of-attack too high to adequately see the landing area and approach aids.

Design of a modern, navalised strike fighter is therefore an almost endless series of compromises. Sadly, each of these compromises will almost inevitably engender embittered opposition from on or another constituency that feels its own particular ox is being gored when a choice goes against them. At some point, honest disagreements on priorities can bubble out of the developmental cognitive bin and into the political oversight space. These eruptions can put the program on the horns of a dilemma: As stated above, “Money is life.” The obverse side of this heuristic is that “Money makes you a target.” When budgets are tight at DoD  (and when aren’t they?)  hungry opponents within the service, in other services, or in the political branches will look for reasons to raid a program to support their own programmatic priorities. Vociferous intramural debates are healthy, but they can also provide outsiders the ammunition they need to threaten or even kill a program. Strong leadership within the Navy succeeded – barely – in exercising cloture over the internal debate, in an attempt to ensure that the “perfect” was not the mortal enemy of the “good enough.” The vendor, doing their own part to ensure program survival, was very careful to ensure that their sub-contractors had presences in nearly every state with a senior Congressional delegation, thereby assuring that budgetary marks against the program would result in hometown pain.

In the course of building a mental model which would adequately address customer needs, design choices were made which would eventually have outsize consequences. The goal of creating a nimble fighter that was also a stable bomber led to redundant flight control computers, while the single-seat design, when combined with multiple mission tasking, necessitated a simple pilot/aircraft interface and intuitive displays – although the aircraft missions and functions were complex, the user interface was designed for simplicity. The heuristic of “complexity within systems, simplicity between them” was well found in the FA-18 program. In fact, through the use of standardized data bus exchange protocols, the airframe was in a state of near-continuous evolution. In the long run, these decisions – some of which had been made as much by necessity as by virtue – had the advantage of conferring a market pleasing “high tech” panache on the FA-18 program. Finally, after the Cold War, with increasing pressure on DoD fiscal accounts, the training and life-cycle savings of a single-seat aircraft as contrasted to marginally more capable multi-seat variants proved not insignificant.

Design Evolution: Conceived as a “low end” strike fighter for legacy carrier decks, the FA-18A (the “odd” lettered aircraft, “B” and “D” were trainers, rather than front line fighters) underwent a lengthy evolutionary process. As the A-12 program slowly died, and the medium attack A-6 went into senescence, the requirement emerged for precision guided munitions capability – the AN/AAS-38 Forward Looking Infrared (FLIR) pod and weapons system met that requirement for laser guided bombs, while digital data busses on the fuselage and wing weapons stations ensured future compatibility with MIL-STD 1553 weapons such as the AIM-120 Advanced Medium Range Air-to-Air Missile (AMRAAM), Joint Direct Attack Munition (JDAM) and the Join Stand-off Weapon (JSOW). As the F-14 and S-3 Viking programs approached sunset, the FA-18E/F design (for the first time, the two-seat “F” variant was an operational item) replaced the Tomcat in Fleet Air Defense through its larger fuel fraction, as well as taking on the residual mission of the Viking in providing air refueling support. All of these design evolutions – from the minor transition of A to C, to the major redesign from C to E/F – succeeded in holding tightly to what had worked well in the system architecture before, while remedying former deficiencies.

Airframe Design: The airframe design was chiefly architected to ensure high angle of attack maneuverability, a pointed weakness in the FA-18′s predecessors. Extensive use of composites and titanium sparring ensured light weight, which serves to increase maximum allowable instantaneous normal load factors, reduce the life-cycle stresses of repeated g-loading, while also favorably decreasing the denominator of the thrust-to-weight calculation. Given the same installed engine thrust, these architectural concepts result in greater instantaneous g-available, and enhanced sustained maneuverability over time. The initial YF-17 airframe had slotted inlets in the leading edge extension, which itself acts as a kind of lifting body. These slots were designed to minimize rudder and vertical tail blanking by the main wingform during high angle-of-attack flight, but adversely affected range specifications – in production FA-18′s they were filled in.

The aircraft was also equipped with high lift devices such as leading and trailing edge flaps. In older aircraft, these devices were deployed mechanically under a rigid airspeed schedule. The incorporation of digital flight control computers, serviced by air data computers, rate gyros and accelerometers enabled a full authority, fly-by-wire Control Augmentation System, or CAS. This permitted the aircraft to smartly deploy the high lift devices, and indeed the primary flight controls, throughout the aircraft’s operating envelope. For example, at very high airspeeds and low altitudes, normal use of aileron would ordinarily result in degraded turn performance due to torsional wing flex – a phenomenon known as “aileron reversal.” In the FA-18, the flight control computers analyze the dynamic pressures in this environment and use differential horizontal stabilator to turn the aircraft. Conversely, at medium or high altitudes and very high angle of attack, use of ailerons to turn the aircraft left or right might very well result in an adverse yaw departure, as the upward going wing in the turn goes suddenly past stall, resulting in a sudden increase in induced drag. In this situation, the computers use coordinated rudder and aileron to turn the aircraft in the direction the pilot has “voted.” These computers enable the jet to perform nimbly in air combat, and sturdily in a dive bombing run, while reducing the number of hazardous loss-of-control incidents.

Additional augmentations were in the use of the clamshell canopy, again a lesson learned from Vietnam. The F-4 Phantom, which had been designed as an over water, high speed, fleet air defense interceptor, sacrificed rearward visibility for enhanced aerodynamics and top-end speed. Employed overland in Vietnam, burdened by external bomb carriage and with onboard radar advantages neutralized by very low targets popping up behind them, this lack of rearward visibility was a significant vulnerability.

The YF-17 and FA-18 both made room for two afterburning engines. In the carrier operating environment, landing opportunities are tightly scheduled, and a single engine jet with a rough running motor can easily end up in the water rather that in the wires if things go from bad to worse. Having had the opportunity to shut down engines operating out of tolerance while many hundreds of miles from the nearest landing field, I can personally attest to the value of having a properly running engine left over after the shut down. Multi-engine reliability also proved crucial in combat, as Marine FA-18′s executing close air support missions in Iraq and Bosnia landed safely after having been hit by portable, infra-red guided, surface-to-air missiles, while their single engine AV-8B colleagues often had to eject from aircraft with similar hits.

Finally, the modular system design of the subcomponents allowed both rapid upgrades as technology emerged, and ready maintainability. These two combined to keep the overall lifecycle costs of the airplane down while keeping it abreast of the developing threat, and were a tipping point in favor of continuing the program as compared to more complex, difficult to maintain and difficult to upgrade aircraft.

Mission Flexibility: Especially after the incorporation of MILSTD digital weapons data busses into the weapons system architecture, the number of weapons the FA-18 can carry is enormous. This offers the Navy compelling advantages in mission flexibility, as aircraft can swiftly be reconfigured from fighter to ground attack or even sea control. It is possible in fact to re-role aircraft actually in flight, as a dedicated bomber can be “swung” into defensive counter-air, for example, should the threat dictate. This mission flexibility could create an enormous problem for single seat aviators, however if their interfaces with the weapons system are not properly architected. While there are intellectual challenges to mastering a great variety of weapons, in the FA-18 itself, complexity is highly compartmentalized within the weapons computers themselves, and display and control mechanizations – weapons computer outputs – are as standardized as is possible. Thus, the “cage/uncage” button on the left throttle serves to 1) cage or uncage the heads-up-display (HUD) velocity vector in the navigation master mode, 2) cage or uncage the AIM-9M seeker head in the air-to-air master mode, and 3) cage or uncage – the phraseology becomes inexact here, but the analogy is consistent – air-to-ground weapons systems seekers. Similarly, the radar slew button atop the right throttle “moves things” on displays, whether they be inertial waypoints in nav mode, radar cursors in air-to-air or weapons seekers in air-to-ground.

Human Systems Engineering: The US Navy has operated many single seat aircraft in its history, with the A-7 Corsair, A-4 Skyhawk and F-8 Crusader as the most recent, pre-Hornet examples. Perhaps unsurprisingly, these airplanes had significantly higher accident rates than their multi-seat contemporaries. The FA-18 program was lucky in that its predecessors existed during a transition period from an “owner’s manual” philosophy of aircraft operation, one emphasizing the machine and its operating limits, into the 1960′s NATOPS program. NATOPS stands for “Naval Aviation Training and Operations Standardization,” and incorporated an increased focus on the human element as the pilot interacted with the technology. The lessons learned from NATOPS had the salutary effect of both reducing overall numbers of mishaps while increasing the rigor of scrutiny into root causes of those that remained.

In previous single seat designs, the array of cockpit instruments, displays, controls, switches and rheostats proliferated alarmingly as new technology was incorporated. In the F-8 cockpit for example, new control consoles were installed wherever they fit. In one memorable example this resulted in a UHF radio being installed on the starboard side console so far behind the pilot’s ejection seat that its orientation was actually reversed, compared to the rest of the control boxes – had it not been, a pilot looking over his shoulder would not be able to read its dials. To change a frequency, the pilot had to release the engine throttle, fly the jet with his left hand and crane his head to look behind him on the console, all the while flying into the unseen unknown at hundreds of miles per hour. Doing so in bad weather or at night or during a carrier approach was obviously a recipe for vertigo and potential disaster.

In the FA-18 by contrast, nearly all the avionics – and all of the weapons systems – can be actuated either by the controls mounted on the stick and throttle, or else on an “Up Front Control” directly before the pilot. Additional displays beyond those needed to fly, navigate and deliver ordnance are nested as sub features within the dashboard-mounted multi-function displays, clearing the cockpit of visual clutter. Apart from the conventional location of flight and engine controls, the landing gear, tailhook and flap actuators were placed in intuitive console areas close to where the pilot’s hands would naturally be when time came to activate them. Single-time actuators such as the Emergency Jettison button was located in an easy to reach portion of the front console, but next to absolutely nothing else – a pilot blowing the external stores off the wing could never explain afterwards that he was “going for the other button.” These human systems engineering functions have the blessing of being blindingly obvious only in retrospect, which is probably a pretty good metric for true genius in architectural design.

Summary: Because of the magnitude of design trade-offs required to perform the multiplicity of required functions, the FA-18 program was almost killed before the first airplane hit fleet parking. Its survival and eventual dominance in the naval aviation tactical inventory is the result of careful architectural design intersecting with seized opportunities leavened by fortuitous circumstance. Political resistance was overcome by clever marketing and a wise use of outsourcing, while a focus on overall life-cycle costs of ownership weighed against mission flexibility proved critical to the program’s success.

Part 1 is here


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Filed under Best of Neptunus Lex, by lex, Carroll "Lex" LeFon, Carroll LeFon, FA-18, Lex, Naval Aviation, Naval History, Neptunus Lex

5 responses to “Homework, part II

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  5. mcthag

    My uncle Bill said you learned to do the radio on the F-8 by touch or you learned to deal with disorientation.

    Considering that many aviators got downright good at bringing the Crusader home…

    He complained more about how touchy the damned engine was way more than he complained about changing freq’s.

    Sadly, he’s as dead as Lex and I can’t ask him now.

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