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.
