AP-2H TRIM (Trails Roads Interdiction Mission)
While the Air Force experimented with laser ranging and television cameras in its Tropic Moon I and II self-contained attack aircraft, the Navy introduced an airplane with a similar purpose, the fruit of that service's Trail-Road Interdiction Multisensor project (an apparent case of the acronym, TRIM, determining the title). The TRIM aircraft carried a formidable array of sensors, all of which saw duty in Air Force gunships - an infrared detector, low-light-level television, radar, the night observation device, and an ignition detector.
The Navy chose to test this gear, much of it handcrafted, in an obsolete aircraft, the Lockheed AP-2H, a variant of the OP-2E, which had already proved unacceptably vulnerable to antiaircraft fire during an abbreviated sensor-dropping career. In 1966 Lockheed started to modify four SP-2H under the TRIM (Trails and Road Interdiction, Multisensor / Mission) program as gunships. The MAD boom was deleted and replaced by a twin 20-mm tail turret. The large APS-20 radome was replaced with the smaller APQ-292. Chin mounted infrared sensors and low light level TV were installed, while the dorsal turret was removed and faired over. Other armament consisted of a 7.62-mm minigun pod that was mounted at a 30 degree down angle and fuselage mounted 40-mm grenade launchers. The multiple Miniguns mounted at various angles in the bomb bay created a spray effect when fired.
The AP-2H Neptune aircraft was flown by Heavy Attack Squadron (VAH) 21. Established in September 1968 in Cam Ranh Bay, Republic of South Vietnam, the squadron was disestablished in June 1969. A total of four SP-2Hs were modified by Lockheed at the Burbank, Calif., facility and accepted into VAH-21. In addition to the armament and sensor systems, the heavily armored planes were fitted with state-of-the-art electronics, air conditioning and a special escape system. No planes were lost in the squadron's brief history, but battle damage from ground fire was a routine problem.
On 01 September 1968 Heavy Attack Squadron 21 became the first squadron in the Navy with a night interdiction mission using new electronic surveillance equipment (multi-sensors). Its mission was to interdict logistics moving over land or sea. A detachment of VAH-21 was established at NAF Cam Ranh Bay, South Vietnam. The detachment at NAF Cam Ranh Bay had been a Naval Air Test Center Project TRIM Detachment (TRIM: Trails Roads Interdiction Multi-sensor) prior to becoming a VAH-21 detachment.
On 16 June 1969, with the disestablishment of VAH-21, its record included no loss of aircraft or any wounds suffered by its personnel during operations in Southeast Asia. Even though enemy gunners drove the AP-2H from the skies over southern Laos, the Navy profited from the experiment, fitting out an interdiction version of the Grumman A-6C Intruder that combined infrared equipment, a radar capable of tracking vehicles, and low-light-level television. The first of the modified Grumman jets began flying armed reconnaissance over the Ho Chi Minh Trail during the southwest monsoon season of 1970, joining other A-6s already in action.
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