Maritime patrol aircraft

A maritime patrol aircraft (MPA), also known as a patrol aircraft, maritime reconnaissance aircraft, maritime surveillance aircraft, or by the older American term patrol bomber, is a fixed-wing aircraft designed to operate for long durations over water in maritime patrol roles — in particular anti-submarine warfare (ASW), anti-ship warfare (AShW), and search and rescue (SAR).

In addition to dedicated airframes, mid-size and large business jets have been modified for MPA missions, offering rapid deployment, extended range, long endurance, and lower life-cycle costs.

Among other maritime surveillance resources, such as satellites, ships, unmanned aerial vehicles (UAVs) and helicopters, the MPA is an important asset. To perform ASW operations, MPAs typically carry air-deployable sonar buoys as well as torpedoes and are usually capable of extended flight at low altitudes.

The first aircraft that would now be identified as maritime patrol aircraft were flown by the Royal Naval Air Service and the French Aéronautique Maritime during the First World War, primarily on anti-submarine patrols. France, Italy and Austria-Hungary used large numbers of smaller patrol aircraft for the Mediterranean, Adriatic and other coastal areas while the Germans and British fought over the North Sea. At first, blimps and zeppelins were the only aircraft capable of staying aloft for the longer ten hour patrols whilst carrying a useful payload while shorter-range patrols were mounted with landplanes such as the Sopwith 1½ Strutter. A number of specialized patrol balloons were built, particularly by the British, including the SS class airship of which 158 were built including subtypes.

As the conflict continued, numerous aircraft were developed specifically for the role, including small flying boats such as the FBA Type C, as well as large floatplanes such as the Short 184, or flying boats such as the Felixstowe F.3. Developments of the Felixstowe served with the Royal Air Force until the mid 20s, and with the US Navy as the Curtiss F5L and Naval Aircraft Factory PN whose developments saw service until 1938. During the war, Dornier did considerable pioneering work in all aluminium aircraft structures while working for Luftschiffbau Zeppelin and built four large patrol flying boats, the last of which, the Zeppelin-Lindau Rs.IV, influenced development elsewhere resulting in the replacement of wooden hulls with metal ones, such as on the Short Singapore. The success of long range patrol aircraft led to the development of fighters specifically designed to intercept them, such as the Hansa-Brandenburg W.29.

Many of the Second World War patrol airplanes were converted from either bombers or airliners, such as the Lockheed Hudson which started out as the Lockheed Model 14 Super Electra, as well as older biplane designs such as the Supermarine Stranraer, which had begun to be replaced by monoplanes just before the outbreak of war. The British in particular used obsolete bombers to supplement purpose-built aircraft for maritime patrol, such as the Vickers Wellington and Armstrong-Whitworth Whitley, while the US relegated the Douglas B-18 Bolo to the same role until better aircraft became available. Blimps were widely used by the United States Navy, especially in the warmer and calmer latitudes of the Caribbean Sea, the Bahamas, Bermuda, the Gulf of Mexico, Puerto Rico, Trinidad, and later the Azores.

A number of special-purpose aircraft were also used in the conflict, including the American-made twin-engine Consolidated PBY Catalina flying boats, and the large, four-engine British Short Sunderland flying boats of the Allies. In the Pacific theatre, the Catalina was gradually superseded by the longer-ranged Martin PBM Mariner flying boat. For the Axis Powers, there were the long-range Japanese Kawanishi H6K and Kawanishi H8K flying boats, and the German Blohm & Voss BV 138 diesel-engined trimotor flying boat, as well as the converted Focke-Wulf Fw 200 Condor airliner landplane.

To finally close the Mid-Atlantic gap, or "Black Gap", a space in which Axis submarines could prey on Allied shipping out of reach of MPAs, the British Royal Air Force, the Royal Canadian Air Force, and the US Army Air Forces introduced the American Consolidated B-24 Liberator bomber, which had a very long range for the era. The B-24 was also used at the basis for the PB4Y-2 Privateer, a dedicated MPA variant adopted in large numbers by the US Navy, which saw service late on in the Pacific theatre.