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The AH-64 was first known as the Hughes YAH-64. The twin-engine, two place attack helicopter was Hughes Helicopter's (now Boeing) entry in the US Army Advanced Attack Helicopter (AAH) competition which ran from 1973 to 1976. The AAH program was initiated to develop an attack helicopter for anti-armor operations in day, night, and adverse weather conditions with emphasis on the helicopter's ability to be based with the troops in the field. The program was begun soon after the cancellation of the Lockheed AH-56 Cheyenne program. The AAH reflected a reorientation in Army thinking based on combat experience in Vietnam. Both competitors for the AAH award, the Bell YAH-63 and the Hughes YAH-64, first flew in September 1975, and two flying prototypes of each were delivered to the U.S. Army for evaluation in May 1976. The Army selected the Hughes design on 10 December 1976. Hughes was awarded a contract to begin a full-scale engineering development program that commenced in 1977.

Developing the advanced attack helicopter (YAH-64) was one of the Army's highest priorities in 1978. The YAH-64 is the first Army attack helicopter designed specifically for day or night, adverse weather, antiarmor missions. It carries a crew of two; the pilot in the rear, the copilot gunner operating the weapons systems from the front seat. Two competing contractors had developed target acquisition designation and pilot night vision systems. In 1978 they completed the design and began the prototype. In September 1978 the first target acquisition designation and pilot night vision systems were delivered to the prime contractor for installation and testing.

A Handling Qualities Evaluation, Engineer Design Test 1, was conducted of the Hughes Helicopter Company YAH-64 advanced attack helicopter from 22 April 1978 through 1 May 1978 at Palomar Airport, Carlsbad, CA (elevation 320 feet). A total of 15 flights were conducted during 21.8 hours (17.4 hours productive). The objectives of the test were to reevaluate certain flight characteristics which were undesirable during Phase 1 testing, and to assess the effect of design changes on aircraft handling qualities. This phase of the aircraft development was not intended to address all of the undesirable flight characteristics uncovered during the Phase 1 testing. Many enhancing characteristics, deficiencies, and shortcomings, reported during the government competitive test (GCT) remained valid.

Engineer Design Test 4 (EDT-4) was conducted using a prototype YAH-64 aircraft. This test was conducted at Palomar Airport, Carlsbad, California, (elevation 328 feet) between 10 and 29 November 1980. Approximately 33 hours were flown during 27 flights. Major changes affecting performance and handling qualities were made to the YAH-64 since the last evaluation (EDT-2). These included a new, digital stability augmentation system, and a redesigned empennage featuring an automatically programmed stabilator and an increased diameter tail rotor. In previous evaluations the flight envelope and scope of test were limited by structural loads and continuous monitoring of those loads was required for all flights. During this test, the flight envelope was much larger and no structural limits were encountered. Hover and level flight performance of the aircraft have been improved since EDT-2. Handling qualities have been greatly improved. The most significant areas of handling qualities improvements were in low-speed flight characteristics, including directional control margins, short-period dynamic stability characteristics (particularly in high speed flight), and in aircraft pitch attitude during approaches and IRP climbs. Three deficiencies were found during this evaluation. The disengagement of the HARS, DASE, and automatic operation of the stabilator, and erroneous activation of the engine out/low rotor speed audio warning tone with failure of the No. 1 generator random failure of the master caution light to illuminate with the illumination of some caution on warning panel segment lights; and the restricted pilot's field of view caused by canopy frame structure during NOE and contour flight.

Originally, the YAH-64 featured a T-tail design with the tail rotor mounted mid-way on the vertical stabilizer . The tail was redesigned during the Phase 2 development process into the low-set, fully movable horizontal stabilizer (stabilator) and high mounted tail rotor seen in the production aircraft. The YAH-64 Apache featured two-place crew seats arranged in tandem, as in the Huey Cobra. The Apache was equipped with a Northrop Target Acquisition and Designation Sight (TADS), with direct-view optics, passive forward-looking infrared (FLIR), low-light-level television, laser range finder with target designator, a laser tracker, integrated with a pilot's night vision sensor (PNVS), and a computerized fire control system. The system also features the Honeywell Integrated Helmet and Display Sighting System (IHADSS) which allows for quick and flexible traget acquisition by both crew menbers. The Apache features lightweight boron armor shields in the cockpit floor and sides that can withstand hits from 23mm high-explosive or armor-piercing shells. The YAH-64, was armed with the XM230E1 30mm chain gun, up to 16 Hellfire missiles, and up to 76 2.75 inch rockets.

The Airworthiness and Flight Characteristics Test Part 1 of the prototype YAH-64 helicopter (S/N 77-23258) was conducted in 1981 at three test sites; Palomar Airport, Carlsbad, California (elevation 328 ft). Bishop Airport, Bishop, California (elevation 4120 ft), and Coyote Flats, California (elevation 9980 ft). Approximately 32 productive hours were flown during 45 flights between 30 May and 17 July 1981. Several design changes affecting performance and handling qualities were made to the YAH-64 since the last evaluation (EDT 4) the most significant of which were the changes in tail rotor rigging, the addition of a third DC electrical bus and the incorporation of a Nap-of-the-Earth/Approach mode of operation for the horizontal stabilator. Level flight performances and vibration levels have been slightly degraded since EDT 4, while handling qualities and out-of-ground-effect hover performance remained essentially the same. Numerous anomalies in the operation of the Digital Automatic Stabilization Equipment (DASE) were experienced which may render some of the handling qualities test results suspect. Two deficiencies (both DASE related) were found during this evaluation. Yaw SAS hardovers, and disengagement of the DASE with failure of the No.2 generator. Nine previously unreported shortcomings (4 DASE related) were found.

The Airworthiness and Flight Characteristics (A&FC) Evaluation Part 2, of the prototype YAH-64 helicopter (S/N 77-23258) was conducted at Palomar Airport, Carlsbad, California (elevation 328 ft). A total of 12 flights were conducted between 8 December and 17 December 1981 and 14.3 productive hours were flown. Prior to this test significant design changes were incorporated in the flight control system, the digital automatic stabilization equipment (DASE) and the stabilator system to correct objectionable characteristics determined during the airworthiness qualification program. Significant improvements in handling qualities were noted since the previous evaluation. Uncommanded control inputs, caused by recentering of the SAS actuators, upon failure or disengagement of the DASE may cause a potentially hazardous situation. The instrument flight characteristics of the YAH-64 were satisfactory in smooth air but have yet to be evaluated in turbulent conditions. Manual programming of the stabilator in rearward flight did not significantly reduce objectional vibration at the pilot's station. Results of this test have shown that both previously reported deficiencies and 14 shortcomings have been corrected. One deficiency not previously observed, was identified: the possibility of a false indication of dual engine failure following a single engine failure.

The Airworthiness and Flight Characteristics Test of the YAH-64 helicopter was conducted, ending in 1982. Seventy flights and 70.4 productive hours were flown. Performance testing assessed the probability of meeting the production contract requirements and consisted of an evaluation of hover, takeoff, level flight, forward flight climb and autorotational descent performance. Handling qualities testing determined compliance with selected research and development contract requirements and included standard stability and control tests, an evaluation of slope landing characteristics and instrument flight capability. Additional tests included evaluation of an uprated engine and an external noise survey. The YAH- 64 now meets the performance requirements of the system specification for the production program; the vertical climb and maximum level flight cruise speeds. Slope landing characteristics were satisfactory up to 9 degree lateral slopes and 10 degree longitudinal slopes.

During AAH (Advanced Attack Helicopter) testing in 1981, the Target Acquisition Designation System (TADS) was undergoing developmental and operational testing at the same time. The schedule did not allow enough time for qualification testing (a development test activity) of the TADS prototype prior to a full field test of the total aircraft system, nor was there time to introduce changes to TADS problems discovered in tests. As a result, the TADS performed poorly and was unreliable during the operational test. The resulting DSARC [Defense Systems Acquisition Review Council] action required the Army to fix and retest the TADS prior to release of second year and subsequent production funds.

Environmental testing of the YAH-64 helicopter was conducted in the McKinley Climatic Laboratory, Eglin Air Force Base, Florida. The US Army Aviation Engineering Flight Activity was responsible for the evaluation of aircraft systems and the US Army Aviation Development Test Activity was responsible for the mission equipment evaluation. The test consisted of 14.4 hours of aircraft operating time between 2 November and 16 December 1981. Testing was accomplished at 125, 70, -25, and -50 F with the aircraft attached to the hangar floor. At each temperature, testing consisted of preflight inspections, APU and engine starts, simulated mission profiles, engine shut down, and maintenance inspections. Nine deficiencies were found which would preclude mission accomplishment: (1) the fire control computer was unreliable, (2) the symbol generator required an excessive warm-up time at cold temperatures, (3) the heading and attitude reference system was unreliable, (4) the environmental control unit failed to provide adequate heating or cooling to the cockpit and avionics bays at -25 F, -50 F, and 125 F, (5) the TADS/PNVS and weapons systems did not function adequately at 125 F, (6) the APU aborted its start sequence because it could not accelerate the accessory gearbox to operating speed at -25 F and -50 F, (7) the hydraulic hand pump was ineffective at -25 F and -50 F, (8) the utility hydraulic manifold allowed the accumulator to bleed off at -25 F and -50 F, and (9) failure of the hydraulic flex lines at cold ambient temperatures.

The Apache received its production go-ahead in March 1982 with initial deliveries beginning in January 1984. First article preproduction flight testing was conducted in Mesa, Arizona ( elevation 1387 feet) between 6 August and 29 August 1984. Twenty test flights were conducted for a total of 32.2 hours (22.1 hours productive). Level flight performance and handling qualities tests were conducted to verify that the production aircraft characteristics are the same as those of prototype YAH-64 as tested in Airworthiness and Flight Characteristics (Part 3) tests. The level flight performance and handling qualities of the production aircraft were not significantly different from those of the prototype. Additionally, the performance data presented in the operator's manual is a good representation of the production aircraft performance. One enhancing characteristic was found (the automatic contingency power feature of the T700-GE-701 engine). Two deficiencies were identified (the false indication of engine failures which was previously reported and the poor engine/airframe response characteristics which was previously reported as a shortcoming). Four shortcomings of the prototype aircraft had been corrected in the production aircraft while 13 remain. Additionally, 13 new shortcomings were found.

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