One Hundred Years of U.S. Navy Air Power (57 page)

Engines

Stepping on the gas of a car yields instant response—the same as when a pilot moves the throttle forward on a propeller-driven airplane. This is because the increased gas flow to the cylinders causes bigger explosions and moves the pistons faster right away. Not so in a jet engine. Increasing the fuel flow causes a hotter fire and higher pressures to be generated in the combustion chambers, but this high-pressure gas must now travel aft to the turbine section and cause it, and the attached compressor section up front, to spin a bit faster. This faster spin compresses the incoming air more, which in turn generates higher pressures in the combustors, which then cause the turbine to spin faster, and so on. Obviously, this takes a finite amount of time. The
heavier the compressor and turbine assembly, the more inertia it has, and the more time required to spin up and get to full power. Early jet engines had heavy rotating cores and lower operating temperatures due to less advanced metallurgy, so the lag time between the pilot putting the power lever forward and full thrust being produced was significant. Slow engine response, coupled with the difficult slow-speed approach and landing characteristics of swept wings, produced a lethal combination for Navy pilots. Some early jets such as the Panther and Cougar had centrifugal flow jet engines in which the compressor sent air outward instead of straight back. A centrifugal flow compressor was heavier and less efficient than an axial flow one, and thus increased engine lag even more.

However, jet engine spool-up lag was only half of the problem; engines also lagged in spooling down when the pilot pulled back on the throttle. In piston-engined propeller aircraft, not only did the engine immediately stop putting out power, the propeller acted as a kind of speed brake. Thus, on the straight deck carriers, the Landing Signal Officer (LSO) would give a “cut” signal when the aircraft had the deck made, and the prop plane would settle into the wires. If, for some reason, the hook did not engage the wires, there was a barrier set up to snag the landing gear and stop the plane before it ran into the aircraft parked on the forward part of the flight deck. This system did not work well for jets. Engine lag tends to bedevil student pilots when they first attempt to fly a jet straight and level, causing over-controlling and a rather sinusoidal flight path until they get the hang of leading the power, both in adding it and reducing it. Combined with the lack of a robust and disciplined transition program from propeller aircraft, which was precisely the case in the late 1940s and early 1950s, jet engine lag set the stage for disaster aboard straight deck aircraft carriers. The straight-winged jets such as the Banshee and Panther tended to “float” anyway, the wings continuing to generate lift even when the aircraft speed was just above stall. As a result, if a pilot added too much power in close to the carrier on the final few seconds of an approach, say, to compensate for going below glidepath (remember, the pilot probably added a little power and got no instant response like in a prop, and so added more), then realized he had over-corrected and pulled the power to idle on the LSO's cut signal, the engine would continue to pump out some residual thrust as it spooled down and the jet would level off (float, especially as it got instantly into the “ground effect” of the carrier's flight deck) and not only miss the wires, but also the mid-ship barriers. Most of the time there were many aircraft parked up on the bow, protected, in theory, by the barriers. The result was a catastrophe in which the floating jet ploughed into the parked machines. This happened more than once. Even with improved jet engines, problems with residual thrust did not entirely disappear, but the arrival of the angled flight deck turned potential catastrophes into harmless “bolters” in which the jet simply continued off the angle and into the air to try again.

As metallurgy and jet engine design improved, response lag was reduced. The J-79 turbojet that powered the F-4 Phantom had very quick response and pilots loved it. Moreover, it proved to be highly reliable. However, a trend developed in the 1960s to place fan jet engines in tactical jets. A fan jet engine has a greater diameter compressor section so that some of the compressed air bypasses the “hot section” of the engine and flows directly out the tailpipe. This adds efficiency to the engine, giving the airplane better “mileage.” For tactical jets, which always lack space for fuel, increased mileage means greater radius of action; a good thing generally. However, there was a price to be paid. Fan jets originated in the airline industry, where the engines are treated tenderly, being brought to full power gradually and left unmolested at cruising power for most of the flight. Not so with fighters. The requirements of combat and carrier landing demand constant and rapid throttle movements. Obviously, the addition of a fan section adds weight to the rotating core, so it is easy to imagine the impact on throttle response times. Moreover, the constant stress of having to accelerate that heavy fan imposes much greater stress on the turbine section. Sure enough, Navy fighters and attack aircraft equipped with fan jets started suffering high rates of engine failure. This was bad enough in a two-engine F-14, but catastrophic in the single-engine A-7 Corsair. At one point, the Navy made Corsair pilots limit their throttle movements so the temperatures on the turbine blades could be controlled. In addition, the engines were being preemptively replaced every two hundred hours of operation. Even with these restrictions Corsairs were falling out of the sky. The TF-30 engine, which powered both the early A-7 models and the F-14, also had a tendency to develop compressor stalls, where the airflow through the compressor burbles, like in a wing stall. The engine bangs like crazy and loses power. Many jets were lost to such stalls.

Man-Machine Interface

Cockpit design—the arrangement of gauges and controls—has always been an issue of intense interest to designers and aircrew. Prior to the introduction of jet fighters, the cockpits of Navy aircraft were relatively simple, consisting of sufficient gauges to exert basic control, and in some cases to “fly blind” in clouds and bad weather. Navigation was generally via a compass and a map. As jets were introduced, so were more advanced electronics systems such as radar, weapons control, and eventually electronic navigation. The operation of these systems required quite a bit of attention and effort by the pilot; so much so that pilot distraction caused more than a few accidents. This was exacerbated by the introduction of surface-to-air missiles, which forced attack aircraft to adopt the tactic of low-level approach to their target. Flying a jet at high speed, close to the ground required intense concentration and a highly disciplined approach to using the various electronic systems. If the pilot spent
just a little too much time focused inside the cockpit trying to update his navigation system or change channels on his radio, he could be annihilated in a cloud of fire and dirt as his airplane hit the ground at high speed. This was the fate of perhaps dozens or scores of aircrew over the years as they struggled to carry out their missions using cockpit systems that required lots of attention. One fix for this problem was to insert another crewmember (Radar Intercept Officer, Bombardier-Navigator, etc.) whose whole job was to manage aircraft systems. While the “crew concept” did indeed improve the operational effectiveness of aircraft with complex systems, it did not eliminate the problem of aircrew distraction altogether.

Perhaps the most demanding environment was the low level nuclear mission, which had to be carried out day or night and in almost any kind of weather. While early jets simply could not operate at low level at night or in bad weather, the introduction of both the A-6 Intruder and the A-7 Corsair was supposed to open up this regime to naval aviation. Crews were forced to train in these conditions, and defects in man-machine interfaces soon produced a number of crashes and fatalities, especially in mountainous terrain, where the planes' radars were supposed to alert the pilot of impending obstacles. The A-6 system proved reasonably effective at keeping the plane from hitting mountains, but less so at avoiding power lines. The A-7 system was judged unsuitable after several fatal crashes in which the pilot under instruction was head down under a view-blocking canopy and chased—for safety—by an instructor in another aircraft. Apparently the terrain following radar did not generate a climb signal in time and the instructor pilot's warning came just a little too late.

Nor was low altitude, high-speed navigation the only regime in which systems design flaws proved fatal. In the age before computers, many basic “housekeeping” tasks had to be handled by the aircrew. Fuel management was a constant source of problems and indeed contributed to the deaths of some early jet test pilots. In most Navy jets from FH-1 Phantom to the A-7 Corsair II, the crew either had to take some action to get fuel to transfer in the right sequence from each of the plane's tanks, or had to go through some sequence of switch flipping if the automatic fuel transfer sequence did not occur correctly. Complicated relay logic and cockpit switch sequences caused any number of jets to quit running with plenty of fuel still aboard. Fuel management headaches were multiplied by the introduction of mid-air refueling. In the A-7 for instance, if the refueling probe was bent, and this was not an uncommon result of trying to get fuel from an Air Force KC-135, fuel transfer from the drop tanks was inhibited, so the Corsair driver could have four-thousand pounds or more of unusable fuel hanging from his jet. Because most jets required hydraulically boosted flight controls to compensate for transonic shockwaves, unreliable hydraulic systems posed similar problems. Thus most jets were equipped with multiple hydraulic systems that commonly required somewhat complicated
“switchology” drills if one or more failed. Again, many airplanes were lost when pilots flipped the switches in the wrong order.

Armed with this mini-education about the hazards of flying swept-wing jets, we can go on to review with greater insight and appreciation the transition from straight-wing piston-engine propeller planes to swept-wing jets on board the Navy's aircraft carriers.

GETTING ON AND OFF THE AIRCRAFT CARRIER WITH JETS

By the end of World War II, the U.S. Navy had become very adept at operating aircraft carriers. It had an extensive cadre of highly experienced pilots that provided leadership in the air wings and squadrons and excellent instruction in the training command, and it knew how to get the air wings on and off the carriers. In the 1920s and 1930s naval aviation had developed technologies that allowed the carriers to operate sixty or more aircraft. Of course, a key technology was the arresting wires stretched across the flight deck of the carrier and the arresting hook attached to the tail of the airplane. However, the real key to operating large numbers of aircraft was the midship barrier, a series of elevated wires that would catch the landing gear of any aircraft that happened to miss the arresting wires or whose hook bounced over them or perhaps just broke. This allowed the ship to park aircraft up on the bow for refueling and rearming without having to send them down to the hangar deck. In addition, naval aviators had devised a circling landing approach that allowed the pilot to observe the LSO who was standing on a small platform well aft on the flight deck on the port side of the ship. Armed with two paddles, the LSO would let the pilot know if he was too high or too low, and gave him a “cut” signal to reduce power to idle when he had the deck made. This whole system worked well for propeller aircraft, with their relatively low approach speeds, light weight, and instant power response.

The first jets had straight wings and were relatively light, so their approach speeds weren't that much different from props. However, given jet engine lag, pilots had to be careful not to pull off too much power when they got a little high on the glide slope. A number of ramp strikes occurred when they did. Conversely, if the pilot jammed on too much power to correct for a low, the jet engine also took its time spooling down, and there were cases, as previously mentioned, of the jet floating over the barrier and crashing into the “pack” of parked aircraft with catastrophic results. The Navy understood that bringing swept-wing jets into the picture would only exacerbate the problems and so delayed the fleet introduction of these machines until several years after they became operational in the Air Force. When the F9F-6 Cougar, a modification of the straight-winged Panther, showed up in squadrons in November 1952, the difficulties of getting it aboard safely were magnified. If a swept wing is dicey to handle at slow speed and wings level, it's doubly so
in an approach turn. In order to make the approach a bit easier for the pilot, the pattern was extended a bit to give him more wings level time in the “groove.” However, this made it harder for the pilot to see the LSO.

Two pieces of British technology came to the rescue for the Navy. The first was the angled flight deck. The flight deck was widened amidships, allowing the landing area to be canted about 10 degrees to port. This permitted aircraft that missed the wires or bounced to add power and go around for another try without crashing into the barrier or the pack of parked airplanes; equally important, it allowed jet pilots to fly a power-on, constant angle of attack approach all the way to touchdown. That way, with the engine already at a relatively high power setting, the lag to attain full power if the wires were missed was minimal. In fact, standard procedure quickly became adding full power immediately on touchdown, arrested stop or not. If the plane caught the wire, the jet would just sit there momentarily at full power, held stationary by the hook. Once stopped, the pilot would reduce power and taxi out of the landing area. USS
Antietam
, an
Essex
-class carrier, was the first to receive this modification and returned to the fleet in 1953. In 1955, USS
Forrestal
, the Navy's first super carrier, expressly designed to accommodate the heavier swept-wing jets, was commissioned with an angled deck. In 1955 another British invention, the optical mirror landing system, was introduced aboard U.S. carriers. This apparatus allowed the pilot to see clearly whether he was on glideslope or not from over a mile behind the ship. Later, the mirror was replaced with a series of Fresnel lenses that performed the same function of providing a visual indication of glideslope, but using much less space. The influence these innovations had on the safety and operational efficiency of aircraft carriers was dramatic: the carrier embarked accident rate per ten thousand landings dropped from thirty-five in 1954 to seven in 1957.
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