One particular online PDF (available from many websites) has a lot of explanatory information amongst a lot of info irrelevant to answering the question above but bear with me to read the text to get an idea of the 'carrier landing problem'. Other specific PDFs such as the LSO NATOPS MANUAL give more specific info with diagrams. However I'll post the material below because I think it is useful... [GS=GlideSlope]
REVIEW OF THE CARRIER APPROACH CRITERIA FOR CARRIER-BASED AIRCRAFT PHASE I; FINAL REPORT 2002http://www.robertheffley.com/docs/HQs/NAVAIR_2002_71.pdf (2.8Mb)
Quotes below from pages 43-65 approximately:
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StabilizationFLOLS stabilization is provided from signals from the ship’s stable element to provide a stabilized GS with respect to the horizon, under moving deck conditions. Two modes of stabilization are available:
a) Line Stabilization: Used as a backup, this mode stabilizes the FLOLS display for pitch and roll motions of the ship to maintain a predetermined line in space at the intersection of the FLOLS light plane and the true vertical plane through the centerline of the angled deck. This stabilizes GS without compensation for ship’s heave.
b) Inertial Stabilization: The inertial stabilization mode is the primary mode of operation for the FLOLS. This mode adds compensation for ship's heave to the line stabilization."
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THE CARRIER LANDINGThe task of landing aircraft at sea on the pitching decks of CV’s has long been recognized as being among the most difficult of aviation's tasks. The environment is certainly among the most demanding encountered anywhere. Advances in technology, reflected in the engines, airframes, control systems, and displays have transformed the hazards. The operational challenge for the technologist is to further improve the safety of landing at sea while increasing the likelihood of successful arrestment on the first attempt.
Designing for CVS entails far more than merely landing on a small dynamic runway at sea.... Several engineering constraints dominate the landing problem. The aircraft must land: 1) in the desired spot in order to engage a CDP, 2) with no lateral drift to stay within the landing area during the run-out, 3) in the proper attitude to set the hook properly in the wires, 4) at an appropriate speed so as to not overstress the arresting gear engine, and 5) within the sink rate limitations of the landing gear. Additionally, the LSO wants to see the aircraft cross the fantail of the ship with a specified margin above the round-down to confidently avoid hitting the ramp ("H/R clearance"). Finally, the combination of flight condition and power response has to be such that full power can be rapidly achieved for successful waveoff or bolter.
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Hook touchdown amidst the wires does not guarantee successful arrestment. Although the hook is heavy, the violence of impacting the deck at such speeds can cause the hook to bounce off of the deck and over any wires. Hydraulic or pneumatic actuators or dampers may resist this phenomenon, but the aircraft must still be close to the appropriate touchdown attitude for them to be effective.
"Hook-skip bolters" commonly result from a last-second nose-down correction intended to save an "over-powered at the ramp" condition. This “play” for the deck succeeds in guiding the aircraft into the wires, but lowers the attitude such that inadequate pressure is applied to the hook-point to keep it low to the deck.
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WOD is the vector addition of the natural wind, and the ship's forward velocity. The WOD is commonly between 25 and 30 kt, thereby significantly lowering the closure rate an equal amount. This represents a substantial decrease from the kinetic energy of the aircraft in the inertial reference frame.
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The modern approach to an angled-deck carrier entails a stabilized GS at constant speed from no less than ½ mile aft of the ship to arrestment. The stabilized GS ensures satisfaction of the touchdown, drift, and H/R constraints.
Flying at a specified approach AOA (in lieu of speed) ensures that the aircraft is in the proper attitude for CDP engagement, and that the approach speed and sink rate are within bounds. Flying a specified approach AOA also provides the pilot and LSO with a consistent sight picture.
Consequently, every aircraft specific approach looks identical, regardless of its weight or external loading. From the LSO platform, or from the cockpit, the picture looks the same day-to-day, regardless of the other variables. Finally, the stabilized approach maintains a moderate nominal throttle setting, permitting fairly rapid response to MIL power in the event of either waveoff or bolter.
The landing task may be made more challenging by the presence of the ship's burble. The source of the burble is the interference of the structure of the ship with the relative wind, and its influence is felt mainly in the last half mile of the approach to the ship. The ship structures that contribute to burble are primarily the island, the bow of the flight deck, and the corner formed between the end of the angle deck and the rest of the flight deck (the "crotch"). Burble consists of random, periodic, and steady components. The random component is chiefly caused by turbulence in the lee of the ship's island structure. This turbulence is worse when WOD are predominantly aligned with the ship's centerline (axial winds), which place turbulence in the lee of the ship's island structure at the in-close position of approaching aircraft. The periodic component of burble is associated, in part, with the cyclic pitching motion of the ship. The steady components of burble consist of a reduction in the steady wind and a predominant upwash aft of the ship that are functions of the magnitude of the WOD, and the range from the ship....
Burble is not considered a major factor in routine shipboard operations, but rather an ever-present feature of the task. Pilots adapt quickly to each ship, subconsciously anticipating the aircraft's reaction to their nominal experience. In rare cases, such as naturally gusting surface winds or
large, rapid ship motion, the burble can have a dramatic effect on the pilot's ability to fly a precise approach. Safety of flight can quickly be compromised, which will result in either a higher accident rate or lower boarding rate.
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PILOT PERSPECTIVEThe pilot is the most unpredictable component of the shipboard landing system. "Fatigue" means something completely different to the aircraft than it does for the pilot. The aircraft flies the same on its 5th shipboard landing as it does 200 later. The aircraft does not care whether the sun is up or down. All of these issues, among others, profoundly affect pilot performance, and consequently system performance. Aircraft features and attributes can limit the variability of the performance of the pilot. This section addresses these topics beginning with the pilot as a multivariable sensor and feedback control system.
The human part of this pilot-aircraft system is limited in the ability to control multivariable problems. A human with sufficient control authority can control one dynamic variable very precisely, two variables precisely, three variables passably. The pilot’s performance deteriorates severely trying to simultaneously control more than three.
Fortunately, the multiple constraints of a CV landing are satisfied by the pilot's control of just three variables – GS, lineup, and AOA. Pilot performance is affected by the allowable tolerance of the accepted deviations, the dynamics of the particular variable, the responsiveness of the aircraft to control inputs, the environmental conditions, and the quality of the information used to determine GS, lineup, and AOA error. It is important to note that tactical Naval Aviators, in the context of CV landings, speak interchangeably about speed and AOA. Though they are reading AOA in their indicators, they refer to themselves as either "fast" or "slow"....
SENSORY INFORMATIONSince the pilot is trying to simultaneously close the loop on three states, the quality of the information provided is vital. GS is provided by the FLOLS that is stabilized in pitch and roll to compensate for pitch and roll movements of the ship. As every aircraft has a different H/E length, the lens is adjusted for each aircraft model, such that if the pilot maintains the central light in the lens aligned with the reference lights ("datum"), the hook will touchdown midway between the second and third CDP. Because the GS information is displayed in a radial fan of light cells, the resolution of the vertical displacement from GS improves with the inverse of distance from the ship.
The centerline stripe and lights provide lineup information, as do the drop lights. AOA is provided internal to the cockpit in a variety of displays. These are the primary sources for the three control states.
It is significant to note that GS, lineup, and AOA in and of themselves only provide displacement error. The best closed-loop performance is achieved feeding back error rates rather than displacement errors themselves. Error rates for GS and lineup can only be assessed by monitoring the change in the error over time. For example, a glance at the lens will identify one's location relative to the GS, but will not identify whether the error is increasing or decreasing. Periodic sampling over some finite time is required to discern whether the GS is improving. The same is true for the lineup. In darkness, LSOs contribute significantly in that they can usually detect developing error rates before the pilot.
Heads-Up Displays (HUD’s), such as that found in F-14D and all F/A-18 models have dramatically transformed the landing problem. First, an Inertial Navigation System (INS)-driven velocity vector precisely displays the projected flightpath of the aircraft. Ashore, the velocity vector permits a pilot to superimpose the symbology directly on the intended point of landing and achieve very precise results. At sea, since the ship is typically moving relative to the inertial frame, the velocity vector does not reliably indicate the point of touchdown. It does, however, provide very precise rate information with respect to GS, with some small bias term. The typical habit for F-18 Hornet pilots is to place the Velocity Vector near the intersection of the decks (“crotch”) of the ship, and then gauge the GS trend. In doing this, the pilot is effectively leading the ship by placing the velocity vector at some point out in front of the wires where the ship and aircraft trajectories will intersect.
This initial placement ensures that the flightpath will very nearly hold the aircraft on GS. The precision of the FPA data also means that the effect of an input correction is immediately assessed in a variable that is very nearly GS rate (the state information necessary for the pilot to attain the elevated performance). As the aircraft approaches the in-close to at-the-ramp position, the velocity vector is allowed to drift aft to the point of touchdown. The fielding of HUD’s largely bears the responsibility for the improvement in boarding rate demonstrated by F-18 Hornets and F-14D model Tomcats over the aircraft that preceded them. One method to reduce pilot workload during the approach task is to improve pilot cueing. Because of the beneficial impact of HUD cueing on pilot workload during the approach task, it is recommended that HUD considerations be a primary consideration in designing for the approach task.
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LINEUPMany of the historical carrier-based accidents are caused by lateral excursions from the landing area during some part of either the landing rollout or a bolter. Since the most common foul-line excursions result in Class 'B' or 'C' (i.e., no loss of crew or aircraft) damage, and lack the dramatic effect of a ramp strike, they do not receive the same attention. Moreover, if the aircraft lands within the wires (within the acceptable GS error band), a successful landing is fairly insensitive to errors in GS rate (sink rate). While the nominal sink rate of 14 fps is prescribed, aircraft loads are certified to sink rates in excess of 20 fps. With respect to lineup, it is not sufficient that an aircraft touchdown within a specified distance of the centerline; it is also vital that the aircraft touchdown with little lateral drift to keep the wingtips within the bounds of the foul line throughout the rollout. Carrier aviation has a lengthy history of aircraft touching down directly on centerline, but then impacting parked or taxiing aircraft with a wingtip during a rollout. For this reason, the backup LSO's principal responsibility is to monitor the lineup of each approaching aircraft. As mentioned previously, lineup control is significantly aggravated by the erosion of sensory information at night, requiring the pilot to devote more attention to lineup to the detriment of the control of the other aircraft states.
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Conclusion: The multiple constraints of CV landing are satisfied by the pilot's control of just three variables - GS, lineup, and AOA. Addition of error rate information improves closed-loop performance. The best closed-loop performance is achieved feeding back error rates rather than displacement errors themselves."
