Precision bombing refers to the attempted aerial bombing of a target with some degree of accuracy, with the aim of limiting collateral damage. An example would be destroying a single building in a built up area causing minimal damage to the surroundings. Precision bombing was initially tried by both the Allied and Central Powers during World War I, however it was found to be ineffective because the technology did not allow for sufficient accuracy. Therefore, the air forces turned to area bombardment, which inevitably brought about civilian casualties. "Precision bombing" was notably attempted by the United States Army Air Forces over Europe during World War II, as it was believed that heavy bombers could accurately bomb targets from high altitude using the Norden bombsight. Since then, the development and adoption of guided munitions has greatly increased the accuracy of aerial bombing. Because the accuracy achieved in bombing is dependent on the available technology, the "precision" of precision bombing is relative to the time period.
Precision has always been recognized as an important attribute of weapon development. The noted military theorist, strategist, and historian Major-General J. F. C. Fuller, considered "accuracy of aim" one of the five recognizable attributes of weaponry, together with range of action, striking power, volume of fire, and portability.
World War II
In the early days of World War II, bombers were expected to strike by daylight and deliver accurately in order to avoid civilian casualties. Cloud cover and industrial haze frequently obscured targets so bomb release was made by dead reckoning from the last navigational "fix"—the bombers dropping their loads according to the ETA for the target. All airforces soon found that daylight bombing resulted in heavy losses since fighter interception became easy and switched to night bombing. This allowed the bombers a better chance of survival, but made it much harder to even find the general area of the target, let alone drop bombs precisely.
The Luftwaffe addressed this issue first by using a series of radio beams to direct aircraft and indicate when to drop bombs. Several different techniques were tried, including Knickebein, X-Gerät and Y-Gerät (Wotan). These provided impressive accuracy—British post-raid analysis showed that the vast majority of the bombs dropped could be placed within 100 yards (91 m) of the midline of the beam, spread along it a few hundred yards around the target point, even in pitch-dark conditions at a range of several hundred miles. But the systems fatally depended on accurate radio reception, and the British invented the first electronic warfare techniques to successfully counter this weapon in the 'Battle of the Beams'
The RAF later developed their own beam guidance techniques, such as GEE and Oboe. These systems could provide an accuracy of about 100 yards radius, and were supplemented by the downward-looking radar system H2S. The British development of specialist 'Earthquake' bombs (which needed to be dropped very accurately) led to the development of supporting aiming techniques such as SABS and the Pathfinder Force. Specialist units such as 617 squadron were able to use these and other techniques to achieve remarkable precision, such as the bombing of the Michelin factory at Clermont-Ferrand in France, where they were required to destroy the workshops but leave the canteen next to them standing.
This development process, driven by the need to bomb in unsighted conditions, meant that by the end of World War II, unguided RAF bombs could be predictably delivered within 25 yards of a target from 15,000 feet height, and precisely on it from low level.
For the U.S. Army Air Forces, daylight bombing was normal based upon box formations for defence from fighters. Bombing was coordinated through a lead aircraft but although still nominally precision bombing (as opposed to the area bombing carried out by RAF Bomber Command) the result of bombing from high level was still spread over an area. Before the war on practice ranges, some USAAF crews were able to produce very accurate results, but over Europe with weather and German fighters and anti-aircraft guns and the limited training for new crews this level of accuracy was impossible to reproduce. The US defined the target area as being a 1,000 ft (300 m) radius circle around the target point - for the majority of USAAF attacks only about 20% of the bombs dropped struck in this area. The U.S. daytime bombing raids were more effective in reducing German defences by engaging the German Luftwaffe than destruction of the means of aircraft production.
In the summer of 1944, forty-seven B-29's raided Japan's Yawata Steel Works from bases in China; only one plane actually hit the target area, and only with one of its bombs. This single 500 lb (230 kg) general purpose bomb represented one quarter of one percent of the 376 bombs dropped over Yawata on that mission. It took 108 B-17 bombers, crewed by 1,080 airmen, dropping 648 bombs to guarantee a 96 percent chance of getting just two hits inside a 400 x 500 ft (150 m) German power-generation plant.
By the time of the Gulf War, the capabilities of "smart" aeroplanes dropping dumb bombs from low altitudes were sufficient to place an unguided munition within 30 feet (9.1 m) of a target. However, Iraqi air defenses in the Kuwait theater of operations, characterized by large numbers of man-portable air-defense systems, surface-to-air missiles and rapid-firing light antiaircraft cannon, simply would not permit such routine use of the low altitude operating environment. Operations from medium altitudes (15,000 ft and higher) at longer slant ranges severely complicated bombing accuracy, particularly against targets that required essentially a direct hit to be destroyed, such as hangars, bunkers, tanks, and artillery. As one analytical study concluded:
Medium and high-altitude bombing with unguided munitions posed problems, even with digital smart platforms. Using smart platforms to deliver dumb bombs against point targets smaller than the circular error probable (CEP) may well require redundant targeting.
Adding to this problem has been a generalized lack of appreciation of how warfare has changed since World War II. On the eve of the Gulf War, for example, critics of proposed military action posited scenarios where tens of thousands of Iraqis would be killed by largely indiscriminate air attacks that would "carpet bomb" population centers, particularly Baghdad.
In the best-known example from the Gulf War, well-publicized attacks against bridges in downtown Baghdad, coupled with a precision attack against the Al Firdos command and control bunker that killed several hundred individuals using it as a shelter, generated a political reaction that included shutting down the strategic air campaign against Baghdad for ten days. The well-publicized precision bombing of the Chinese embassy in Belgrade led to an international outcry, riots in Beijing, and serious international diplomatic repercussions between the People's Republic of China and the United States. The CIA admitted it directed the operation, claiming a targeting error occurred, and both the aircrew and equipment were cleared of blame.
Historical experience through Vietnam
The precision weapon, within generalized boundaries, will perform roughly equally well in all circumstances, provided a target can be identified. Time scales may change and levels of effort may change, but the end result—a victory for the force making the best use of precision—is unlikely to change unless other factors (such as loss of national will, changing international support, "wild cards," etc.) enter play. The single most important factor is how well the decision-maker, both military and political, appreciates what precision weapons can and cannot accomplish, what mechanism or process has been established to assess the appropriateness of their use, and the rules of engagement that govern their use.
Historical experience with precision guided munitions dates back over fifty years; there is a considerable body of historical experience that suggests how precision weapons have dramatically transformed military affairs. The precision weapon era dates to May 12, 1943, when a Royal Air Force Liberator patrol bomber dropped a Mk. 24 acoustic homing torpedo that subsequently seriously damaged the U-456, driving it to the surface where it was subsequently sunk by convoy escort vessels. On September 9, 1943, a German Fritz-X radio-guided glide bomb dropped from a Dornier Do 217 bomber sank the modern Italian battleship Roma as it steamed towards Gibraltar. Two months later, an anti-shipping missile launched from a Heinkel He 177 sank a British troopship with the loss of 1,190 American soldiers, one of the greatest of all maritime disasters. By war's end, Germany and the United States had employed various proto-smart weapons in combat, including radio, radar, and television-guided bombs and missiles, against targets ranging from industrial sites to bridges and enemy shipping.
Although not often thought of as a precision weapon, the various Kamikaze attackers that first appeared in the fall of 1944 functioned much like modern anti-shipping missiles, and thus can legitimately be considered a part of the precision weapon story. The Kamikaze was the deadliest aerial anti-shipping threat faced by Allied surface warfare forces in the war. Approximately 2,800 Kamikaze attackers sunk 34 Navy ships, damaged 368 others, killed 4,900 sailors, and wounded over 4,800. Despite radar detection and cuing, airborne interception and attrition, and massive anti-aircraft barrages, a distressing 14 percent of Kamikazes survived to score a hit on a ship; nearly 8.5 percent of all ships hit by Kamikazes sank. As soon as they appeared, then, Kamikazes revealed their power to force significant changes in Allied naval planning and operations, despite relatively small numbers. Clearly, like the anti-shipping cruise missile of a later era, the Kamikaze had the potential to influence events all out of proportion to its actual strength.
The need to destroy precision targets such as bridges had driven development of rudimentary guided bombs in the Second World War, and Korea accelerated this interest. In Korea, Air Force B-29's dropped the Razon and the much larger and more powerful Tarzon guided bombs on North Korean bridges, destroying at least 19 of them. The disappointing Korean bridge-bombing experience stimulated the Navy to pursue development of the postwar Bullpup program, the first mass-produced air to surface guided missile.
Accompanying this interest in Anti-Surface Warfare, was an equivalent drive to develop precision air-to-surface and surface to surface weapons for antishipping roles. In particular, the Soviet Union pursued development of such weapons as a means of countering the tremendous maritime supremacy of the Western alliance during the Cold War. One of the most significant events in the history of precision weaponry occurred on October 25, 1967, when the Israeli destroyer Eilat, patrolling 15 miles (24 km) off Port Said, was sunk by four Soviet-made Styx antishipping missiles fired from an Egyptian missile boat, killing or wounding 99 of its crew. The sinking of the Eilat had profound impact; one surface warfare officer remarked that "it was reveille" to the surface Navy." One senior American naval officer called the potential Styx threat his "worst nightmare."
The Soviet Union's alarming investment antiship missiles stimulated a tremendous investment in countermeasures. It influenced the purchase of the Grumman F-14 Tomcat, as well as more advanced airborne and surface early warning radars and fire control systems, and new gun and surface to air missile systems. But despite such corrective measures, the problems posed by newer generations of weapons continue to confront naval planners in the present day. Indeed, it can be argued that, at best, defensive measures have kept up with the threat, not surpassed it.
As the antishipping missile transformed war at sea, the advent of the laser-guided bomb revolutionized precision land attack, for it could function with an average circular error of less than twenty feet from the aim point. With this kind of accuracy, the need to operate mass flights of aircraft against a single aim point at last disappeared; it was as revolutionary a development in military air power terms as, say, the jet engine or aerial refueling. Even more significantly, an aircraft dropping a laser-guided bomb could drop it from outside the majority of an enemy's air defenses, thus further reducing the likelihood of incurring losses to enemy defenses. The modern precision weapon era may be said to have begun in May 1972, when laser-guided-bomb-armed F-4 Phantoms perfunctorily took down the Paul Doumer and Thanh Hoa bridges in North Vietnam, as part of a larger campaign that shattered North Vietnam's invasion of South Vietnam in the spring of that year.
Precision attack in the Gulf War
The first Gulf War showed how radically precision attack had transformed the traditional notion of running a military campaign and, especially, an air campaign. On opening night of the war, attacks by strike aircraft and cruise missiles against air defense and command and control facilities essentially opened up Iraq for subsequent conventional attackers. Precision attacks against the Iraqi air force destroyed it in its hangars, and precipitated an attempted mass exodus of aircraft to Iran. Key precision weapon attacks against bridges served to "channelize" the movement of Iraqi forces and create fatal bottlenecks, and many Iraqis, in frustration, simply abandoned their vehicles and walked away. Overall, postwar analysis indicated that Iraq's ability to move supplies from Baghdad to the Kuwaiti theater of operations had dropped from a total potential capacity of 216,000 metric tons per day over a total of six main routes (including a rail line) to only 20,000 metric tons per day over only two routes, a nearly 91 percent reduction in capacity; all others (including the railroad) had essentially been destroyed. What shipments did occur were haphazard, slow, and carried in single vehicles that were themselves so often destroyed that many Iraqi drivers simply refused to drive to the KTO. This destruction had taken place in an astonishingly short time; whereas, in previous non-precision interdiction campaigns, it often took hundreds of sorties to destroy a bridge, in the Gulf War precision weapons destroyed 41 of 54 key Iraqi bridges, as well as 31 pontoon bridges hastily constructed by the Iraqis in response to the anti-bridge strikes, in approximately four weeks.
In the Gulf War, only 9 percent of the tonnage expended on Iraqi forces by American airmen were precision munitions. Not quite half of this percentage—4.3 percent—consisted of laser-guided bombs, credited with causing approximately 75 percent of the serious damage inflicted upon Iraqi strategic and operational targets. The remaining precision munitions consisted of specialized air-to-surface missiles such as the Maverick and the Hellfire, as well as cruise missiles, anti-radar missiles, and assorted small numbers of special weapons. It was, overall, the laser-guided bomb that dominated both the battlefield, the counter-air campaign against Iraqi airfields, strikes against command and control and leadership targets, and the anti-bridge and rail campaign. As the Gulf War Air Power Survey concluded,
"Against point targets, laser-guided bombs offered distinct advantages over "dumb" bombs. The most obvious was that the guided bombs could correct for ballistic and release errors in flight. Explosive loads could also be more accurately tailored for the target, since the planner could assume most bombs would strike in the place and manner expected. Unlike 'dumb' bombs, LGB's released from medium to high altitude were highly accurate. . . . Desert Storm reconfirmed that LGB's possessed a near single-bomb target-destruction capability, an unprecedented if not revolutionary development in aerial arfare."
In particular, the advent of routine around-the-clock laser bombing of fielded enemy forces in the Gulf War constituted a new phase in the history of air warfare. These attacks were not classic close air support, or battlefield air interdiction, but, instead, given the level of accomplishment over time, went far beyond the levels of effectiveness traditionally implied by such terms. Indeed, the vast majority were made in the 39 days prior to the ground operation when the coalition's land forces were, for the most part, waiting for their war to begin. Yet the Iraqi army was, in effect, mortally wounded in this time. These attacks, against Iraq's mechanized formations and artillery, can best be described as a form of strategic attack directed against unengaged but fielded enemy forces, what might be termed DEA: "Degrade Enemy Army." The combination of laser-guided bombs from F-111F's and F-15E's, together with Maverick missiles using imaging infrared thermal sensors fired by A-10's and F-16's were devastating, as were laser-guided bombs from British Tornadoes and Buccaneers, and AS-30L laser-guided missiles fired from French Air Force Jaguars. Particularly deadly were F-111F night "tank plinking" strikes using 500 lb (230 kg). GBU-12 laser-guided bombs. On February 9, for example, in one night of concentrated air attacks, forty F-111F's destroyed over 100 armored vehicles. Overall, the small 66-plane F-111F force was credited with 1,500 kills of Iraqi tanks and other mechanized vehicles. Air attacks by F-15E's and Marine A-6E's in the easternmost section of the theater averaged over thirty artillery pieces or armored vehicles destroyed per night.
Once attack helicopters attached to surface forces entered battle, they demonstrated that such results were not limited to fixed-wing attackers. At sea, Royal Navy and U.S. Navy helicopters destroyed numerous Iraqi small boats and military craft; fourteen of fifteen British Aerospace Sea Skua antishipping missiles launched from Westland Lynx helicopters hit their targets, a hit rate of over 93 percent. French, British, and American gunships destroyed numerous Iraqi mechanized vehicles. McDonnell AH-64A Apache crews of one U.S. Army aviation brigade destroyed approximately fifty Iraqi tanks in a single encounter. Another Apache unit scored 102 hits for the expenditure of 107 Hellfire missiles, a hit rate of better than 95 percent. (Overall, Apache gunships destroyed nearly 950 Iraqi tanks, personnel carriers, and miscellaneous vehicles).
The reaction of Iraqi forces to direct precision air attacks indicated that the traditional powerful psychological impact of air attack had, at last, been matched by the equally powerful impact of actual destruction. What can be identified can be targeted so precisely that unnecessary casualties are not inflicted upon an opponent. In short, war, the great waster of human life, is now significantly more humane. Increasingly, war is more about destroying or incapacitating things as opposed to people. Further, in the precision engagement era, what has changed most dramatically has been the time scale and level of effort required to achieve decisive effects over an opponent. Today, planners are far less concerned about the number of sorties required to destroy a target; rather, they emphasize the number of targets destroyed per sortie as the metric that must be considered.
Reaffirmation of the Gulf experience
Nor was the first Gulf War an isolated example. From August 30 through September 14, 1995, for the first time in its history, NATO forces engaged in air combat operations, against Bosnian Serb forces in the former Yugoslavia during the Bosnian War. A total of 293 aircraft based at fifteen European locations and operating from three aircraft carriers flew 3,515 sorties in Operation Deliberate Force, to deter Serbian aggression. Somewhat less than 700 of these sorties targeted command and control, supporting lines of communication, direct and essential targets, fielded forces, and integrated air defenses. A total of 67 percent of all such targets engaged were destroyed; 14 percent experienced moderate to severe damage, 16 percent light damage, and only 3 percent were judged to have experienced no damage.
In contrast to the Gulf War, the vast majority of NATO munitions employed in the Bosnian conflict were precision ones: in fact, over 98 percent of those used by American forces. American forces employed a total of 622 precision munitions, consisting of 567 laser-guided bombs, 42 electro-optical or infrared-guided weapons, and 13 Tomahawk Land Attack cruise missiles. American airmen dropped only 12 "dumb" bombs. Precision weaponry accounted for 28 percent of NATO munitions dropped by non-US attackers. Sorties by Spanish, French, and British strike aircraft dropped 86 laser-guided bombs, and French, Italian, Dutch, and United Kingdom attackers dropped 306 "dumb" bombs. Overall, combining both the American and non-American experience in Bosnia, there were 708 precision weapons employed by NATO forces, and 318 non-precision ones; thus precision weaponry accounted for 69 percent of the total employed in the NATO air campaign. Combined statistics of American and NATO experience indicate that the average number of precision weapons per designated mean point of impact (DMPI) destroyed was 2.8. In contrast, the average number of "dumb" general purpose bombs per DMPI destroyed was 6.6. The average number of attack sorties per DMPI destroyed was 1.5.
As a result of NATO's first sustained air strike operations, all military and political objectives were attained: safe areas were no longer under attack or threatened, heavy weapons had been removed from designated areas, and Sarajevo's airport could once again open, as could road access to the city. More importantly, the path to a peace agreement had been secured. In total, for an overall expenditure of approximately 64 weapons per day—69 percent (44) of which were precision weapons—NATO forces achieved their military and political objectives. The leverage that this weaponry gave over Balkan aggressors and the recognition of what precision air attack means to decision-makers in the modern world was enunciated by former Assistant Secretary of State Richard Holbrooke after the conclusion of the campaign and the settlement of the Dayton Peace Accords:
"One of the great things that people should have learned from this is that there are times when air power--not backed up by ground troops--can make a difference."
The key, of course, is in the targeting, for air power is like other forms of military power projection in one important regard: namely, to achieve best effect, it must be used overwhelmingly, as part of a well-thought-out strategy, and not merely as a tool of diplomatic signal-sending. Put another way, if the foe is targeted effectively, the foe will get the message. This has been, of course, one of the great concerns we have seen in the current air campaign over Yugoslavia and Kosovo, namely whether the air effort being expended is consistent with what prudent use of air power teaches us from the past.
Holbrooke's statement hints at one of the major effects of precision, namely that the traditional notion of massing a large ground force to confront an opponent, particularly on a "field of battle" is now rendered archaic. To a degree, throughout military history, the span of influence of ground forces was always spreading out the battle area at the expense of "mass." As the zone of lethality an individual soldier could command increased, the spacing between soldiers expanded as well. But the precision attacker overcomes the expansion of the linear battlefield by exercising the ability to undertake individual targeting at ranges far in excess of even the most powerful artillery. Thus airplanes, "smart" ballistic missiles, or cruise missiles, launched hundreds of miles away from a front-line, can then pass beyond that front-line for a distance of hundreds of miles more before targeting some key enemy facility or capability that directly influences the success of enemy operations at the front itself. This is true flexibility, of a sort again unknown to previous military eras. Further, it speaks to a changing paradigm in warfare: from the classic infantry-armor paradigm to a paradigm emphasizing remote fires: air and artillery.
Precision attack versus light infantry
As hinted by the Balkan experience, both in 1995 and in 1999, the advantages of precision attack are not limited to what might be termed "traditional" encounters between massive deployed forces possessing large and vulnerable weapons such as ships, tanks, and vehicles. Indeed, recent examinations of air power applications against light infantry in typical Third World crisis conditions indicate that precision offers very high leverage whether one is dealing with a mechanized force, a guerrilla-type army in a wooded or jungle environment, or, even, an individual urban sniper à la Sarajevo. The combination of new and enhanced sensor technology, coupled with information exchange between targeting systems and strike aircraft, helicopters, or smart missiles, can defeat threats that, in previous times, were considered too difficult to thwart without greatly widening a war effort.
Even light infantry forces generate by their operations and equipment a variety of detectable signatures—visual, chemical, infrared, electromagnetic, radar, and acoustic—that render them vulnerable to a range of active radar sensor systems (such as synthetic aperture, moving target indicator, and foliage penetrating radars), and passive air (and air-deployed ground-based) sensors (such as low light level TV, thermal imagers, multispectral analyzers, engine electrical ignition, and magnetic field detectors). These signatures betray the location and, indeed, strength of enemy forces, enabling targeting systems to then direct air attacks against them.
The capabilities of new detection systems are remarkable by the standards of previous conflict. One countersniper ballistic analyzer, the Lifeguard sniper location system developed by the Lawrence Livermore National Laboratories, detects a sniper's bullet after the round has been fired, analyzes its flight path, and then establishes the bullet track back to its point of origin, all virtually instantaneously, and with an accuracy of within 2 feet (0.61 m) of where the sniper is actually located. If multiple analyzers are present, this track can be refined to within one inch. With this capability, even a sniper operating in the midst of a crowded urban environment is not immune to reprisal—for example, a helicopter gunship firing its cannon on precise coordinates, or a strike aircraft releasing a laser-guided soft and lightweight sticky foam bomb that could burst in a room and kill or disable a sniper without damaging or endangering the surrounding structure or building inhabitants.
Precision weaponry and the revolution in military affairs
In recent years, much has been made about the so-called "Revolution in Military Affairs," and whether, or not, an "RMA," in fact, really is underway. If one truly considers the implications of precision attack, it is clear that precision weapons, when coupled to the other great revolutions of this aerospace century, have transformed warfare, and, as a result, the question is not really one of "Does an RMA exist?" but, rather, "When did it begin, and what are its implications?" Tied to this, of course, are equally-surprisingly persistent questions about the use and value of air power, now more accurately considered as aerospace power. If nothing else, given the record of precision air power application, aerospace power advocates should not still have to spend as much time as they do arguing the merits of three-dimensional war and the value of precision attack to it. Modern joint service aerospace forces offer the most responsive, flexible, lethal, and devastating form of power projection across the spectrum of conflict, employing a range of aerospace weaponry such as maritime patrol aircraft, attack and troop-lift helicopters, land-based long-range aircraft, and battlefield rocket artillery systems. Service-specific aerospace power can often be formidable and, as such, over not quite the last ninety years, has transformed conflict from two-dimensional to three-dimensional, and has changed the critical focus of conflict from that of seizing and holding to one of halting and controlling.
In this regard, it is worth quickly reviewing a few salient points from the military history of this century. Within roughly a decade of the first flight of an airplane, aircraft were having an occasionally decisive effect on the battlefield. Within four decades, a nation—Great Britain—secured its national survival through air warfare. By the midst of the Second World War, three-dimensional attack (from above and below the surface) had become the primary means of sinking both vessels at sea and destroying the combat capability of armies on land. In fact, for the United States, this trend of inflicting losses and material destruction primarily through air attack continued into the postwar years for Korea, Vietnam, the Gulf, Bosnia, and other, lesser, contingencies. In particular, air attack directed against land forces has been especially powerful in blunting and destroying opponents on the offensive, whether in older experience—such as confronting Rommel in the Western Desert, or Nazi armored forces trying to split the Normandy invasion at Mortain, or at the Bulge (where German commanders credited Allied fighter attacks on fuel trucks and supplies as being the decisive factor in halting their drive), in the opening and closing stages of the Korean War, and confronting the 1972 North Vietnamese Spring Invasion—or, more recently, in destroying the Khafji offensive of Saddam Hussein in 1991. NATO's reliance upon air power in the present Balkan crisis should not be surprising, for, from the very earliest days, the NATO alliance saw air power as the linchpin of Western military strength, and the necessary off-set to the Warsaw Pact's huge military forces.
Given its historical underpinnings, we should not be surprised that the revolution in warfare that has been brought about both by the confluence of the aerospace and the electronic revolutions, and by the off-shoot of both—the precision guided munition—is one that has been a long-time coming, back to the Second World War, back, even, to the experimenters of the First World War who attempted, however crudely, to develop "smart" weapons to launch from airships and other craft. Used almost experimentally until the latter stages of the Vietnam War, the precision weapon since that time has increasingly come to first influence, then dominate, and now perhaps to render superfluous, the traditional notion of a linear battlefield.
- The following is an abridged transcript of a speech given by Dr. Richard P. Hamilton, SES, at Eglin AFB, for the USAF Air Armament Summit, on 26 May 1999.: This article incorporates public domain material from the United States Government document "http://www.airforcehistory.hq.af.mil/EARS/Hallionpapers/precisionweaponspower.htm".
- Paul Brickhill, The Dam Busters, 1951, pp.160-161.
- Paul Brickhill, The Dam Busters, 1951, p. 142.
- Hastings, Max Bomber Command Pan (1970)