Culprits of Lockerbie a treatise Concerning the Destruction
Design of the PFLP-GC barometric trigger (switch)
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- The other purpose: much greater lethality at higher altitude
- The time taken for the barometric trigger to activate the capacitor
- The 750mb legend on the barometric switch of the Toshiba RT-F453D bomb seized by the BKA on 26 October 1988
- Possible inconsistency in BKA technical expert Gobel’s opinion on the capacitor run-time for the seized devices
- Challenging Rainer Gobel’s findings on capacitor run-time
- Time to detonation of Lockerbie bomb consistent with use of a PFLP-GC barometric trigger with normal range time-delay
- (i) Barometric bomb designed to skip a leg
- (ii) Malfunction on the first of two legs
Design of the PFLP-GC barometric trigger (switch) Modern small barometers usually comprise a micron sized bar of silicon separating a reservoir of gas from the barometer’s environmental atmosphere. (As such they are technically known as solid state pressure transducers.) In response to a change of relative pressure between the reservoir gas and the environmental atmosphere the dimensions of the bar change and this will be registered electronically on a gauge. Used as a switch, the change can be harnessed to close a micro circuit at a preset position. Such sophisticated switches were unavailable to the PFLP-GC bomb makers in the late 1960s when the barometric bomb was being developed. If the RT-453D Toshiba bomb seized by the BKA on 26 October 1988 is anything to go by it can be asserted that the PFLP-GC used a simple mechanical consumer pressure sensor from an anaeroid barometer. Such a barometer consists of a vacuum chamber sealed by a diaphragm which moves against a spring when the external, ambient, air pressure falls and causes a needle to rotate round a dial. The PFLP-GC adapted such barometers for use in their bombs by placing an electrical contact at a given position on the mechanical dial corresponding to the pressure in the fuselage at which it was desired to close the circuit. When the needle touched the contact as it rotated with the falling pressure a circuit was closed, causing a current to flow from the batteries. In the original design of aircraft bombs built by the PFLP-GC the current flowed directly to the detonator. Thus as soon as the pressure in the fuselage dropped to the pre-set level the bomb would detonate. (b) The first PFLP-GC barometric bombs The first use of a PFLP-GC barometric bomb took place on 21 February 1970 when it destroyed a Swissair Convair CV-990 Coronado four-engine jet airliner en route from Zurich to Tel Aviv with the loss of all on board. Similar bombs were planted on other aircraft during the early years of that decade. (c) Airport vacuum chambers The Swissair bomb and subsequent incidents led to the installation at some airports (including Frankfurt) of vacuum chambers in which suspect luggage could be subjected to lowered atmospheric pressure under controlled conditions. Any explosion would damage only the chamber 
Fig 6 Relationship of altitude to atmospheric pressure 100 kiloPascals = 100millibars (Courtesy of Wikipedia) and its contents. The obvious weakness of this measure was that it was clearly impracticable for busy airports to subject each item of luggage to vacuum chamber scrutiny for more than a minute or two at most. (d) The modified version with time-delay facility Basic form and design To the introduction of the vacuum chamber counter-measure PFLP-GC technicians responded with an ingenious modification in the design of their barometric bomb. Whereas in the original version the closing of the circuit and the flowing of an electrical current from the 1.5 volt AA batteries had immediately activated the detonator, there was now inserted into the chain of operation a simple electrolytic capacitor-type analogue timer. This was predicated to run for a time built into it at the PFLP-GC’s workshop in Damascus. After being tested it was encased in clear resin, which gave rise to the soubriquet “ice-cube” timer. So long as the circuit was closed by the barometer switch as a result of decreased atmospheric pressure a tiny trickle charge would be delivered to the electrolytic capacitor through a resistor. The resistor, as its name implies, restricts the flow of current into the capacitor. The higher the resistor the slower will be the rate of charge and the longer will it take to charge the capacitor to its full potential extent. However, electrolytic capacitors are inherently imperfect and tend to leak electricity as they are charged. The resistor must not therefore be so high in value as to result in the flow of electricity to the resistor being less than the leakage from the capacitor. In that event the capacitor would of course never be charged. Provided the resistor’s value is lower than would cause that to happen the capacitor will gradually be charged to full capacity at a rate dependent (a) upon the resistor value (coded as coloured bands on its tubular body) and (b) upon the applied voltage. On reaching full charge the capacitor would then discharge via a transistor with a burst of current into the detonator. If the aircraft descended and the barometer switched off the circuit before the capacitor was fully charged the charging would cease and the bomb would not be detonated. It was devices of this type that were seized by the BKA from the suspected PFLP-GC German cell in October, 1988, and April, 1989. They were thoroughly examined by the BKA’s chief technical expert Rainer Gobel, who established that they had to be primed, or armed, by inserting a small jack plug into a socket. We shall examine later the ramifications of the cessation of charging of the capacitor before it was fully charged. The other purpose: much greater lethality at higher altitude Not only did the introduction of the timer-delay offer the advantage of circumventing the effects of the vacuum chamber but – very importantly – it also allowed the aircraft to climb to a much greater altitude where the enhanced differential between the artificially maintained pressure within the fuselage and the much reduced external atmospheric pressure would allow the explosion to cause damage to the fuselage framework and skin significantly more lethal than would have been the case at a much lower altitude. Indeed, this may have been a more important reason for the modification than the introduction of vacuum chambers at airports. Given the relatively small size of the device which destroyed Pan Am 103 the terrorists might have been expected to set a time for detonation, if they could, when the aircraft had reached its highest altitude in order to ensure the maximum pressure differential and the most damaging effect of the explosive. (e) Conjecturing the operation of the Lockerbie bomb The effect of fuselage pressurisation With the very great altitudes to which modern commercial airliners ascend it is essential that their fuselages are internally pressurised at a level which simulates the atmospheric pressure at a relatively low altitude. Otherwise no one on board could survive the flight. The mechanically increased pressure inside the fuselage is characteristically measured in terms of the altitude at which that level of pressure would be the natural atmospheric pressure. Hence it is referred to as the “equivalent effective cabin altitude” or “cabin altitude.” The cabin altitude of large jets scheduled to cruise at about 12,200 metres above sea level (40,000 feet approx) or higher is normally programmed to rise from the altitude of the airport of origin to a maximum of around 2,400m (7,800 feet). It remains constant at that level until the aircraft descends and then reduces until the cabin altitude is the broadly same as the real altitude. When the ground air temperature is 15 degrees centigrade the atmospheric pressure at 2,400 metres will normally be about 768mb. This means that on a flight cruising at about 40,000 feet the pressurisation system will be deployed to ensure that the air pressure within the fuselage (ie cabin and hold) eventually drops down to about 768mb but rarely lower, the equivalent of the atmospheric air pressure at an altitude of around 2,400 metres (7,800 feet). Regulations do however allow for a minimum of 750mb, a figure which may bear some significance in the present context. For the purposes of achieving the greatest comfort for all on board the system can be programmed to start pressurisation at or very shortly after take-off and to permit the maximum cabin altitude of about 7,800 feet – that is, the ultimate minimum cabin pressure of 768mb – to be reached only gradually as the aircraft climbs. (Equally, it ensures gentle reduction of the cabin altitude during descent.) The result is that the cabin altitude will not reach its maximum of 7,800 (ie the stage at which the pressure in the fuselage has dropped to its ultimate minimum of 768mb) until the real altitude has reached the final cruising altitude of 40,000 feet plus. The cabin altitude of the Boeing 747 normally reaches about 5,000 feet at a real altitude of about 34,000 feet on a curved rate of increase, and then increases at a slower linear rate. It is important to note that there is no fixed procedure on initiating the system after take-off. Practice varies and is subject to the aircraft captain’s discretion. The system is computer controlled but can be manually adjusted to start before, at or almost immediately after take-off or its onset can be delayed and the rate of change adjusted to the captain’s individual preference. The time taken for the barometric trigger to activate the capacitor On a standard ascent, to what altitude will a Boeing 747-100 climb by the time the gradually reducing internal fuselage pressure has dropped to the point at which the barometric trigger activates the circuit? This will of course depend upon (a) the position selected on the pressure dial at which the contact point for closing the circuit is placed, (b) the time from take-off at which the pressurisation process is programmed to start, and (c) the rate at which it lowers the pressure in the fuselage. The BKA’s chief technical expert, Rainer Gobel, suggested in his report of the examination he made of the Toshiba RT-453D BomBeat cassette radio player seized on 26 October 1988, and basing his findings on documents supplied to him relating to the Boeing 747-100, that the barometric trigger would have activated the circuit at a pressure of 950mb about 7 minutes after take-off (see transcript of his Zeist evidence, p.8794). The normal climb rate of a Boeing 747 series is between 1,800 and 2,500 feet per minute up to 10,000 feet and 2,000 fpm up to 16,000 feet. Thereafter its climb rate drops to 500 fpm until it reaches its ceiling. Taking an average of 2,150 fpm from take-off a 747 will reach 10,000 feet in 4.65 minutes. Continuing to climb at the reduced rate of 2,000 fpm, over the next 2.35 minutes to 7 minutes from take-off it will ascend a further 4,700 feet, making the altitude reached at that point some 14,700 feet. The next 1,300 ft (to 16,000 ft) will take 13/20 minutes (ie, 1,300 divided by 2,000 minutes), or 39 seconds. The ascent thereafter from 16,000 to 31,000 ft at 500fpm will take 30 minutes, making a total of 7 minutes 39 seconds plus 30 minutes. This is almost exactly the 38 minutes Pan Am 103 was in the air before it was destroyed at 31,000 feet, showing that its actual performance on 21 December 1988 matched the standard climb rate. It is also noteworthy that the first airliner known to have been downed by a PFLP-GC barometric bomb – the Swissair Convair CV-990 Coronado four-engine jetliner destroyed en route from Zurich to Tel Aviv on 21 February 1970 with the loss of all on board – was at 14,000 feet when the bomb detonated, although it had been in the air for 9 minutes. Assuming that the barometric trigger had the same pre-setting as that in the RT-453D examined by Rainer Gobel the contrast with the above calculations for the 747 may be explained by a different climb rate and pressurization start time and rate of increase for the Coronado compared with the 747 on PA103. The real altitude at which the natural atmospheric pressure is 950mb is 500 metres, that is, 1,625 feet. Gobel’s calculations must have presupposed a cabin altitude of 1,625 feet (that is 950mb of cabin air pressure) at a real altitude of 14,700 feet 7 minutes from take-off on a standard climb rate pattern. Heathrow lies at an average of 81.25ft (75 metres) above sea level. To reach a cabin altitude of 1,625 feet in 7 minutes from a starting altitude of 81.25 ft would mean an average rate of cabin altitude increase of 1,544 feet divided by 7, that is, 220 feet per minute from take-off. This would have been consistent with, and within the parameters of, a normal pressurisation programme for the 747 (see eg http://www.airliners.net/aviation-forums/ tech_ops/read.main/183281/). The 750mb legend on the barometric switch of the Toshiba RT-F453D bomb seized by the BKA on 26 October 1988 The barometric switch in the RT-F453D bomb seized by the BKA on 26 October 1988 bore a legend “750mb.” This presented something of a puzzle for the Lockerbie investigators because if the switch was designed to trigger at that pressure it would never have turned on the circuit since the pressurisation system would not normally allow the fuselage pressure to drop that far. The actual pressure level at which the switch was set to work at some point could have been no lower than the minimum of 768mb. (In fact, of course, as we have already noted, Gobel found that it would have started charging the capacitor at 950mb.) The investigative journalists Steven Emerson and Brian Duffy in their book The Fall of Pan Am 103:inside the Lockerbie investigation (New York: Putnam, 1990, p.267) cited a senior Scottish official as saying that investigators now believed that the 750mb mark was intended to remind PFLP-GC carriers not to transport the bombs at elevations above 9,000 feet. This, Emerson and Duffy observe, was strange, since none of the Alpine passes that could be traversed by car was above 8,500 feet. The point was not well made. The normal altitude corresponding to an atmospheric pressure of 750mb is 2,500 metres, or 8,125 feet. However, it was noted earlier that flight regulations permit a barometric minimum in pressurised cabins of 750mb. Could the PFLP-GC technicians have designed the pressure switch to accommodate that as the minimum setting at which it could close the circuit? On reflection this seems unlikely. If the pressure switch setting could be extended to close the circuit at such a relatively low pressure (ie, taking into account the effects of pressurisation, only after the aircraft had been in the air long enough to reach a very high real altitude) there would have been no need to introduce a capacitor-type time delay device into the design. Although the 750mb marking remains a mystery it may be conjectured that the barometer from which it was adapted by the PFLP-GC was originally marketed as a relatively low level altimeter for hikers in mountainous regions of Southern Germany and that the 750mb marking merely represented the lower pressure on the dial, the atmospheric pressure at 2,500 metres, that is just over 8,000 feet. Possible inconsistency in BKA technical expert Gobel’s opinion on the capacitor run-time for the seized devices If (a) taking account of the normal flight plan of a Pan American 747-100 Rainer Gobel was correct in his assumptions about the barometric switch in the RT-453D seized by the BKA on 26 October 1988 (triggered at 950mb 7 minutes after take-off), (b) an identical barometric trigger had been used for Pan Am 103, and (c) the barometric switch was coupled with a capacitor-type time delay device, this would have meant that the capacitor discharged at 31 minutes after the trigger closed the circuit and it began to be charged. This would have been the time from take-off to detonation (38 minutes) less the 7 minutes calculated by Gobel to have been the time from take-off to the moment at which the switch closed the circuit. However, it is important to be aware of a potential inconsistency in his findings on this point. During cross-examination by the defence at Zeist, Gobel stated (see transcript, p.8795-8796): “[I]n the first explosive case examined in the Toshiba BomBeat, I measured a time, a pure delay time, of between 35 and 45 minutes. In a different explosive charge, where it was not possible to measure the time [and] it had to be calculated on the basis of the resistors, it was possible to establish a shorter time, of about 35 minutes. So that as far as I am concerned, it is certainly within the realm of the possible that such tolerances – that is the possibility between the different designs to make sure that an explosion would have been possible up to 38 minutes after take-off. It is not possible to prove it, but the technical options are such that it is possible.” If Gobel’s 7 minutes for the barometric switch to activate the circuit are added to the minimum time to detonation based on his measurement of “between 35 and 45 minutes” for the “pure time delay”, that is to say 35 + 7 minutes, this would be four minutes longer than the 38 minutes from Pan Am 103’s take-off to detonation over Lockerbie. This would amount to an inconsistency in his calculations. However, that is not the end of the story. If, in keeping with permissible – indeed common – practice, the Maid of the Seas captain had delayed initiating pressurisation for a certain period after take-off and in the event not until the plane had climbed to at least 500 metres (1,625 feet) above sea level the barometric switch would have closed the circuit (950m being the normal atmospheric pressure for that altitude). At a mean climb rate of 2,150 fpm it would have taken 43 seconds to reach 1,625 feet (ascending 1,544 feet from Heathrow’s average altitude of 81.25 feet). The 38 minute total flying time to detonation minus the 43 seconds to the barometric switch closing the circuit would leave roughly 37 minutes for the timer. This would of course be well within Gobel’s parameters (between 35 and 45 minutes) for a bomb identical to the Toshiba BombBeat seized by the BKA and, importantly, would not have been inconsistent with pilot discretion over when to initiate pressurisation. Challenging Rainer Gobel’s findings on capacitor run-time It is not clear on precisely what basis Rainer Gobel predicated his report on the seized BomBeat. Did he merely rely on a close visual examination of its components, in particular the barometric switch and the capacitor delay-timer and its associated resistors, together with interpretation of 747-100 documentation, or did he also subject them to workshop testing? It is of some interest to compare Gobel’s opinion as to the run time of the capacitor in the seized BomBeat, that is between 35 and 45 minutes, with technical findings reached by Dr Jim Swire, whose daughter Flora was on board Pan Am 103 and who has long been an indefatigable and highly knowledgeable campaigner on behalf of the British relatives of the victims of the atrocity. In correspondence with the present author Dr Swire described having constructed several simple analogue capacitor timers of the type that were used in the seized BomBeat, using resistors of varying values. It has already been mentioned that capacitors are imperfect and leak electricity as they are charged. If the value of the resistor is too high the rate of leakage will exceed the rate of charge and the capacitor will never be charged. Using the highest value resistor short of that which exceeded the leakage rate Dr Swire found it was impossible to attain a run longer than a maximum of approximately 30 minutes, even for those in which he used the most modern components. Accordingly, he found it difficult to accept Gobel’s higher run-time estimates. In Dr Swire’s considered opinion Marwan Khreesat, the PFLP-GC’s bomb-maker who may well have been responsible for building the fatal device, could not have increased the run-time beyond 30 minutes without thoroughly re-designing the timer and using a digital model and, as Dr Swire points out, there was no evidence that Khreesat – whose technical background was essentially in television repair – knew how to construct digital circuits, there being an order of magnitude between analogue and digital circuitry use. Dr Swire suggests that there may be hints in Gobel’s phraseology that he deduced run times of the various timer units he examined from the markings on the components visible from outside the resin rather than from actual repeated bench testing of each unit. In the passage of his cross-examination referred to above (Zeist transcript at pp.8795-8796) the translation of the verb Gobel used was given as “measured” which in German can be either messen or abmessen, the latter of which can also be translated as “gauge” or “estimate”. On the other hand, he did contrast his “measurement” of the pure delay time in the case of the BomBeat with his approach in relation to “a different explosive charge” (presumably the Sanyo monitor) in respect of which it was not possible to measure the time and he had to calculate it on the basis of the resistors. On the face of it this would seem to imply measurement of the BomBeat delay actually by trickle-charging the capacitor. Dr Swire notes that electrolytic capacitors are subject to a good deal of performance variability and uncertainty, are brutally affected by ambient temperature and can have a value as much as 20 per cent above or below that printed on the device. Reducing by 20 per cent the lowest figure on Gobel’s estimated range for the seized Toshiba, that is 35 minutes, we arrive at 29 minutes. If as regards the run-time of the timer Dr Swire is right (and Gobel was mistaken) in asserting that it could not have exceeded 30 minutes, this would almost exactly reconcile Gobel’s opinion (assuming pressurisation began very soon after take-off) that the barometric switch in the seized Toshiba would have activated the timer after 7 minutes (when the pressure was 950mb) with Pan Am 103’s 38 minutes from take-off to detonation at an altitude of 31,000 feet. Time to detonation of Lockerbie bomb consistent with use of a PFLP-GC barometric trigger with normal range time-delay One prominent commentator on the Zeist trial noted that the judges were “so impressed” by the “amazing coincidence” that barometric pressure triggers of the kind used in the bomb seized by the BKA will cause an explosion 38 minutes after take-off on a standard ascent, and the explosion of the Pan Am103 bomb after 38 minutes into the flight, that they . . . ignored it (Foot, op cit, p.25). (f) RARDE supposition that a primed barometric bomb could have skipped the Frankfurt-Heathrow leg Is it conceivable that a PFLP-GC-type barometric bomb was smuggled aboard the feeder flight PA103A at Frankfurt, that it did not detonate on the flight to Heathrow but did so on board the main flight PA103 to JFK? Two possibilities might be conjectured here. (i) Barometric bomb designed to skip a leg One possibility which might be conjectured is that the PFLP-GC bomb makers came up with a design of barometric device which allowed the mechanism to stay inert for the first leg of a journey but would then automatically detonate a bomb on a subsequent leg. Whether it might have been feasible to design such a bomb, there was no evidence that any of the PFLP-GC bombs recovered in Autumn Leaves (and assumed to be Khreesat’s handiwork) incorporated any circuitry necessary for such a purpose. In any event it was not part of the Crown’s case that the Lockerbie bomb was a barometric controlled device, relying as they did on the supposition that it was operated by a stand-alone digital clock timer, the Mebo MST-13. Moreover, little purpose would be served by speculating on how such a device might have worked for the simple reason that its employment would have required a convoluted action plan involving a wholly unnecessary degree of risk of failure. (ii) Malfunction on the first of two legs The second possibility which might be conjectured is that there was some sort of malfunction in the mechanism which caused the detonation to fail on Pan Am 103A but to succeed after the bomb was transferred to Pan Am 103. In a review of Rainer Gobel’s findings Allen Fereday of RARDE suggested that such a second possibility was not inconceivable. It is necessary to examine how this might have occurred. Directory: docs docs -> Application for acem docs -> Observational Assessment of Teaching Practices Teaching Assessment Initiative Proposal Submitted to The Teachers for a New Era project docs -> Traditional British values docs -> From Warfighters to Crimefighters: The Origins of Domestic Police Militarization docs -> Borzoi Club of America Register of Merit Program I. 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