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A maritime patrol aircraft (MPA), also known as a patrol aircraft, maritime reconnaissance 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).
The first aircraft that would now be identified as maritime patrol aircraft were flown by the Royal Naval Air Service during World War I, primarily on anti-submarine patrols. At first, blimps and zeppelins were the only aircraft capable of staying aloft for the long periods of time (as much as 10 hours) needed by the patrols whilst carrying a useful payload. Shorter-range patrols were mounted by adapted landplanes such as the Sopwith 1½ Strutter. Later in the war, aircraft were developed specifically for the role. These were usually large floatplanes such as the Short 184 or flying boats such as the Felixstowe F.2. The U.S. Navy's Curtis NC long-range floatplanes were also designed and developed as ASW patrol aircraft, but they did not enter service until the war was over. In 1919, the Curtis NC-4 became the first aircraft of any kind to fly across the Atlantic Ocean, via New York City, Nova Scotia, Newfoundland, the Azores, and Lisbon, Portugal, and then continuing on to Southampton, England taking 19 days, proving the long-range capabilities of aircraft. Crossing the Atlantic in a zeppelin came later on.
Many of the World War II patrol airplanes were converted from either long-range bombers or airliners (such as the German Focke-Wulf Fw 200 "Condor"). U.S. Navy blimps were also widely used, especially in the warmer and calmer latitudes of the Caribbean Sea, the Bahamas, Bermuda, the Gulf of Mexico, Puerto Rico, Trinidad, and the Azores, when those became available.
To cover some of the Mid-Atlantic Gap, or "Black Gap", that existed before the Azores became available in mid-1943, the British RAF, the Royal Canadian Air Force, and the U.S. Army Air Forces employed the "Very Long Range" version of the American designed and produced Consolidated B-24 Liberator bomber. This aircraft was capable of patroling the areas around Nova Scotia, Newfoundland, Greenland, Iceland, Bermuda, Scotland, Morocco, Trinidad, Puerto Rico, and the Bahamas with long radii of operation, to seek out and attack German U-boats on the surface. The new developments inairborne radar enhanced this, especially at night and in the cloudy/foggy weather of the North Atlantic
Special-purpose aircraft were also used, including the American-made twin-engined Consolidated PBY Catalina flying boats, and the large, four-engined British Short Sunderland flying boats of the Allies. For the Axis Powers, there were the long-range Japanese Kawanishi H6K and Kawanishi H8K flying boats, and the German Blohm & Voss Bv 138 trimotor flying boat. Also, in the War in the Pacific, the Catalina was superseded by the better Martin PBM Mariner longer-ranged flying boat.
In the decades following World War II, the patrol duties were partially taken over by aircraft derived from civilian airliners. These had range and performance factors better than most of the wartime bombers. The latest jet-powered bombers of the 1950s did not have the endurance needed for long, overwater patrolling, and they did not have the low loitering speeds necessary for antisubmarine operations.
The U.S. Navy flew a mixture of patrol planes such as the Lockheed P2V Neptune (P2V) and the Grumman S-2 Tracker, carrier-based, and then replaced the P2V with the Lockheed P-3 Orion (P-3A, P-3B, and P3C), still in service after many decades, derived from the modern Lockheed Electra airliner with four turboprop engines. The P-3 has been produced in California and in Japan and Canada, and it has also long served in the air forces and navies of Japan, Canada, Australia, Brazil, Germany, Holland, New Zealand, Norway, Spain, and the Republic of China (Taiwan, ROC). The Canadian version is called the CP-140 Aurora.
At first, the Royal Australian Air Force and the Royal Australian Navy had to made do with a stretched-fuselage modification of the Avro Lincoln bomber, before replacing those with the P2V and then the P-3C, still in service.
In addition to their ASW and SAR capabilities, most P-3Cs have been modified to carry Harpoon missiles and Maverick (missile)s for attacking surface ships if the need ever arose. (It never has, nor has a P-3 ever attacked an enemy submarine.) American P-3s were formerly armed with the Lulu nuclear depth charge for ASW, but those were removed from the arsenal and scrapped decades ago.
The Soviet Union developed the Ilyushin Il-38 from a civilian airliner, and the Royal Canadian Air Force did likewise, deriving the Canadair CP-107 Argus from a British airliner. Then, as mentioned above, those were replaced by the CP-140 Aurora, derived from the Lockheed Electra.
The French Navy developed the Breguet Atlantic following a Request for Proposal (RFP) from the North Atlantic Treaty Organization (NATO). Some of these were also produced for some other NATO members that were not flying the P-3 or the CP-140.
Japan developed a purpose-designed aircraft as well, the Shin Meiwa PS-1 flying boat.
The main threat to NATO maritime supremacy throughout the 1960s, 1970s, and the 1980s was Soviet Navy and Warsaw Pact submarines. These were countered by the NATO fleets, the NATO patrol planes mentioned above, and by sophisticated underwater listening systems. These span the so-called "GIUK Gap" of the North Atlantic that extends from Greenland to Iceland, to the Faroe Islands, to Scotland in the United Kingdom, which locations also have the main air bases for NATO patrol planes: U.S. Navy and Canadian ones from Greenland, Iceland, and Newfoundland; British ones from Scotland and Northern Ireland; and Norwegian, Dutch, and German ones from their home countries.
Since the end of the Cold War the threat of a large-scale submarine attack is a remote one, and many of the air forces and navies have been downsizing their fleets of patrol planes. Those still in service are still used for search-and-rescue, counter-smuggling, antipiracy, antipoaching of marine life, the enforcement of the exclusive economic zones, and enforcement of the laws of the seas.
The earliest patrol aircraft carried bombs and machine guns. Between the wars the British experimented with equipping their patrol aircraft with the COW 37 mm gun. During World War II, depth charges that could be set to detonate at specific depths, and later when in proximity with large metal objects replaced "anti-submarine" bombs that detonated on contact.
Patrol aircraft also carried defensive armament which was necessary when patrolling areas close to enemy territory such as Allied operations in the Bay of Biscay targeting U-boats starting out from their base.
As a result of Allied successes with patrol aircraft against U-boats, the Germans introduced U-flak (submarines equipped with more antiaircraft weaponry) to escort U-boats out of base and encouraged commanders to remain on the surface and fire back at attacking craft rather than trying to escape by diving. The advantage was short lived as the submarine was defenceless if it tried to dive for long enough for the aircraft to make its attack, effectively preventing it from diving until a surface ship could arrive to destroy it. Equipping submarines with radar-warning devices and the snorkel made them harder to find.
To counter the German long-range patrol aircraft that targeted merchant convoys, the Royal Navy introduced the "CAM ship", which was a merchant vessel equipped with a lone fighter plane which could be launched once to engage the enemy planes.
Maritime patrol aircraft are typically fitted with a wide range of sensors:
A modern military maritime patrol aircraft typically carries a dozen or so crew members, including relief flight crews, to effectively operate the equipment for 12 hours or more at a time.