The All-Weather Carrier Landing System

The modern attack aircraft carrier, with her steam catapults, angled deck, stabilized optical lens landing systems, and advanced electronic developments of the computer age, has been limited operationally by adverse weather conditions when ceiling and visibility are less than instrument minimums. Also, despite improved Carrier Controlled Approach (CCA) techniques, increased reliability of radio equipment, and modernized visual landing aids, the carrier air wing’s landing accident rate has not improved proportionately during the past few years.

Landing accidents account for nearly one- half of all naval aviation accidents. And, a type of landing accident which continues to occur at an alarming rate is the “ramp strike”—an aircraft dropping below the glide path and striking the approach end of the flight deck. Furthermore, the night carrier landing accident rate has remained about triple the day rate. A common cause factor in these night carrier landing accidents is the inability of the pilot to position his aircraft properly for the landing because of the absence of customary visual cues. The night landing technique also requires the pilot to alternate from instrument to visual flight in the late, critical stages of the carrier approach.

Thus there has been an urgent requirement to improve the attack carrier’s operational posture by eliminating weather restrictions to the landing phase and reducing the landing accident rate. Early efforts in this area employing precision approach equipment, similar to that used at shore installations, were relatively unsuccessful, because the pilot had to have sufficient time to orient on the optical landing system prior to completing his approach. There was also the problem of the ceiling and visibility being below the minimums required for use of the optical landing system in weather conditions of zero ceiling and zero visibility.

During 1950, the Bureau of Ships initiated the development of an automatic landing system, later designated the All-Weather Carrier Landing System (ACLS). A contract was awarded to the Bell Aerosystems Company for a landing control central radar, the AN/SPN-10, which it was hoped would enable carrier aircraft to be landed safely and automatically in all types of weather, day or night. The first automatic touchdown by an aircraft using this system was made ashore in 1954. A fully automatic shipboard landing was made by an F3D Skynight on board the USS Antietam (CVS-36) in September 1957. This test was made with the use of special, hand-tooled equipment.

Following the successful Antietam tests, the Bureau of Ships ordered production of the SPN-10 equipment, and it has since been installed in the attack carriers Midway (CVA-41), Independence (CVA-62), Ranger (CVA-61), Franklin D. Roosevelt (CVA-42), America (CVA-66), and Kitty Hawk (CVA-63). Installation is proceeding on board the Enterprise (CVAN-65). Shore-based SPN-10 units are located at the Naval Air Station, Miramar, California, and the Naval Air Test Center, Patuxent River, Maryland.

The essential elements consist of both shipboard and airborne equipment (Figure 1). The SPN-10 radar includes two identical precision tracking radars which illuminate a preselected volume in space known as the “acquisition window.” As the aircraft passes through this window, the radar automatically acquires and tracks the aircraft’s corner reflector. The instantaneous position co-ordinates of the aircraft with respect to the deck touchdown point are supplied by the radar to the data stabilization unit where the position co-ordinates are corrected for ship motion to establish aircraft position with respect to the true vertical and horizontal. These stabilized position co-ordinates travel to the flight path computer where they are compared with the desired flight path to establish vertical and horizontal error signals. Using these error signals, the flight path computer calculates and generates pitch and bank commands, required by the auto-pilot of a preselected type of aircraft, to direct the approaching aircraft to the desired flight path. During the last 12 seconds before touchdown, the deck motion compensation unit furnishes additional signals to the flight path computer to modify the commands to the aircraft. The control commands and the error signals are encoded and transmitted over the data link. The data link information is received in the aircraft and decoded. Flight-path error signals are then sent to the cockpit display, while pitch and bank commands are coupled to the aircraft’s auto-pilot and are used to maintain the aircraft on the desired flight path to the touchdown point on the carrier flight deck.

The SPN-10 has been designed to provide fully automatic carrier landings under the following conditions:

Three modes of landing operation are provided by the ACLS: automatic (Mode I), semiautomatic (Mode II), and manual or precision-approach-type talk-down (Mode III). In Mode I, the landing aircraft is controlled to touchdown by its automatic flight control system (auto-pilot) in response to SPN-10 command signals, without pilot assistance.

For Mode II approaches, the flight path errors determined by SPN-10 are displayed in the cockpit of the approaching aircraft and the pilot maneuvers the aircraft, using instrument landing system techniques, to reduce the displayed errors to zero until Mode II minimums (ceiling and visibility) are reached.

During Mode III operation, flight path correction commands are issued verbally by the console operator over voice-radio link and the pilot is talked down, using Ground Controlled Approach procedures, to precision approach minimums. When approach minimums are reached in both Mode II or III, the pilot must convert to the optical landing system and complete his landing or pull up and execute missed-approach procedures.

The aircraft pilot, in all cases, selects the mode of operation and may, at his discretion, terminate the landing sequence at any time.

The SPN-10 console operator also monitors the aircraft’s position on the glide path in Modes I and II.

The landing control central SPN-10 provides preset landing programs for up to ten different aircraft types. These programs are based on the desired glide slope, the flight characteristics of the aircraft, and the height above the deck of its corner reflector at touchdown. Glide slope and corner reflector height program data may be set into the equipment manually by the console operator to automatically land additional types of aircraft.

“Data link” is the term used to denote a system of high-speed communication using digital intelligence and automatic messageprocessing techniques. The ACLS uses the Naval Tactical Data System data link which sends separate messages to different aircraft over a common, ultra-high-frequency channel by transmitting the messages sequentially. Since each channel has a capacity for many messages per second, time division permits sharing the message capacity of the system among as many aircraft as desired. In order that the displays and auto-pilot flight control systems in each aircraft may accept only error signals and control commands intended specifically for that aircraft, each pilot sets into his airborne data link an assigned address so it will accept only messages sent to his aircraft and reject all others.

The F-4 Phantom II pilot’s cockpit display provides two types of information, the flight path error signals displayed on vertical and horizontal cross bars and pilot advisory discrete words, also generated by the SPN-10, sent by data link, as follows:

CHECK LIST

ACLS READY FOR LOCK-ON UNDER AUTOMATIC CARRIER LANDING (ACL) CONTROL 

10 SECONDS UNTIL TOUCHDOWN 

WAVE-OFF

A fail-safe feature in the aircraft causes the discrete word “tilt” (Transmissions for Intercept and Landing Terminated) to be displayed whenever the data link information is unreliable.

In order to realize the ACLS Mode I touchdown dispersion, it is essential that the speed of the approaching aircraft be maintained to plus or minus two knots of the predetermined approach speed. The approach power compensator or control, an automatic throttle control, is required for ACLS Mode I operation airspeed control, but functions independently. This device presents a new concept in the control of engine thrust during the landing mode. The aircraft’s angle of attack transducer is modified to provide electrical signals which are combined in an amplifier with the output of a normal accelerometer. The amplified signals then operate an integrated torque boost unit which is mechanically coupled to the engine throttle linkages. Engine thrust is controlled automatically by the approach power control during the final approach to touchdown by signals generated in the angle of attack transducer. This relieves the pilot of one of his normal duties during the carrier approach, in that the aircraft’s desired angle of attack is automatically maintained and the result is a nearly constant airspeed.

The final approach and landing are monitored at all times by the landing signal officer, the SPN-10 console operator, and the aircraft pilot.

A typical ACLS Mode I approach starts with the aircraft being guided into the acquisition window by TACAN navigation or radar landing traffic control. The pilot completes his landing check list, trims his aircraft for level flight, and engages the approach power compensator. One SPN-10 radar is placed in automatic search mode, generating an acquisition window in space along the deck center- line. As the aircraft flies through the window, the radar locks onto the aircraft. The second radar takes over the search function to acquire the next aircraft. Discrete lock-on signals are transmitted to the aircraft by data link. The pilot engages his auto-pilot and acknowledges “pilot ready” by data link reply message. Upon receipt of “pilot ready,” commands are transmitted and the aircraft is under completely automatic control. The aircraft converges with the preselected ideal glide path and pitches over to follow the constant glide path maintained in space coordinates. Deck motion compensation begins approximately 12 seconds before touchdown. A “ten-second” advisory discrete word message is sent to the aircraft advising the pilot that touchdown is imminent. After the aircraft lands, the radar returns automatically to the search mode and begins the acquisition cycle for the next aircraft.

The ACLS concept has been proven by more than 5,000 successful, fully automatic landings both on land and during shipboard evaluations under test conditions. An operational evaluation for ACLS Mode III operations was completed in mid-1963 on the Midway. Results showed that Mode III talk- down approaches can be safely made under reduced ceiling and visibility minimums. A successful Mode II Technical Evaluation was completed early in 1964 using the F-4G Phantom II aircraft.

A major program to equip most carrier- based aircraft with an ACLS capability will extend over four years. A one-way data link (receive-only) will be used instead of the larger, heavier, more-complex, and more- expensive two-way data link. The “pilot ready” message will be transmitted by the pilot verbally over the radio approach control circuit in this case.

Future ACLS project plans include Mode I technical and Mode II operational evaluations late in 1965, and Mode I operational evaluation in 1966. Earliest Fleet introduction of aircraft equipped for ACLS Mode I operations is planned for late 1966, coincident with completion of shipboard installations.

A carrier air wing equipped with ACLS will be able to land safely and automatically in all types of weather, day or night, bringing a new dimension to attack carrier operations.