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Aviation History
1960
1960 - 2243.PDF
FLIGHT, 7 October 1960 AVIATION ELECTRONICS . . . 579 S.T.C. RADIO ALTIMETER FOR THE VC10 CPECIAL radio altimeters are required for automatic flare-out^ and landing, and Standard Telephones and Cables Ltd were responsible for the development, in conjunction with the BlindLanding Experimental Unit, of the first altimeter specifically designed for use in automatic landing. The first of these units wasthe STR.30 and its derivatives which employed a pair of horn aerials whose apertures were flush with the aircraft skin andcovered by plastic plates. Subsequently S.T.C. have produced a new range of radio altimeters, the STR.41 and STR.40, thelatter being designed to indicate only between 0 and 500ft and intended specifically for automatic landing. An STR.40 weighs 201b and includes a pressurized containerfor transmitter/receiver, amplifier and counter, two horn aerials with aerial matching units attached and a delay line unit providinga predetermined height signal for functional checking. A control unit and indicator are available, but need not be used if thealtimeter is integrated with the automatic controls. Transmitter and modulation frequencies are respectively 4,200 to 4,400Mc/sand 300c/s. Minimum height reading is 2ft above the runway and zero error not greater than lft at touchdown. A functional checkwith the delay line can be carried out at any height and provides immediate overall calibration. The STR.40 is transistorizedexcept for the transmitter stage. The aerials are of robust con- struction and, S.T.C. say, do not require periodic servicing. Clearly, the altimeter must be integrated with the autopilot andprovided with the same degree of redundancy as the other elements in the overall system. For the VC10, Elliott Brothers (London)Ltd have specified that each radio altimeter should incorporate an additional sub-receiver element, in parallel with the main receiver,the sub-receiver being coupled to the common receiver aerial by means of a measured length of coaxial cable. This effectivelyincreases the height measured through the sub-receiver; and a continuous logical comparison assessment can then be madebetween the main receiver and sub-receiver outputs, the difference between which should always be equal to the constant heightequivalent of the coaxial cable. Failure of either receiver element results in a change in thedifference signal, while failure of the transmitter or aerials or any common element will result in the output signals of the tworeceivers either going to zero or becoming incoherent. In each case, the change in the difference signal is used to indicate thefailure and disconnect the autopilot. A pair of isolated signal The S.T.C. STR.40 radio altimeter on which the VC10 installation will be based. Two altimeter systems will be carried in each VC10, each being monitored in the way described here, and each passing height signals to one of the autopilots outputs from the main receiver and a disconnect line are requiredto match the unit to the VC10 system philosophy. The VC10 will, of course, be equipped with two radio altimeter systems of thiskind, one serving each of the two autopilots. The technique described eliminates the need for a third altimeter with itsassociated aerials. A number of detailed designs, based on the STR.40, are now under investigation. The illustration above shows the basic STR.40. In the versionapplied to the VC10, a matching unit is needed only on the receiver aerial and the output from the delay line feeds straightinto the monitor sub-receiver. Thoughts on Scheduled Automatic Landing by G. S. Bishop* FOR unscheduled automatic landing, it has already been shownthat there is a basic requirement for two complete autopilotchannels to permit continued operation in the event of any single fault. Additionally, either a third channel or monitoringequipment is needed to enable faulty equipment to be unambiguously located and disconnected. Automatic landingequipment will therefore be between two and three times as complex as a simple unmonitored autopilot, and might be expectedto have an overall MTBF of at mest 500hr. To meet the required standard of safety, the whole equipment must be functioning atthe 200ft break-off height before the pilot can select an automatic flare-out or landing; otherwise he must make a manual landing.Over a five-hour flight, there is therefore a one per cent probability that a conventional landing will be necessary, and the implicationis that this is possible, i.e. either that the visibility at some airfield within range is sufficient for a manual landing or that other aids,such as GCA, are available and adequate. Scheduled automatic landing implies that no such stand-by isavailable, so that the decision to perform an automatic landing is made not at the break-off height, but at take-off. The critical timeis thus not 30sec, but the whole flight time and, because the probability of a single fault during flight is one per cent, theprobability of a double fault is i X10'4. Although only a proportion of such double faults will put the system out of action, it is clearthat, in order to meet the ARB proposed requirement, scheduled automatic landing equipment will require sufficient redundancyto survive at least any two faults, and possibly more. * Transport Aircraft Controls Division, Elliott Brothers (London) Ltd. There is, however, an intermediate stage between unscheduledand scheduled automatic landing which might be called "part- scheduled" automatic landing. Although the system proposed forunscheduled automatic landing is sufficiently safe for blind land- ing provided that it is fully serviceable at the break-off height, aheavy jet transport may at this point have insufficient fuel reserves to climb out and divert to a clear airfield. The committal point forsuch an aircraft is at about 15,000ft, so that the critical time is about half an hour. Assuming an autopilot MTBF of 500hr, the probability of asingle fault during this time is 10~3 so that the possibility of using unscheduled automatic landing equipment for part-scheduledautomatic landing requires further investigation. The committal point is the point beyond which the pilot no longer has the optionof landing manually, which means that he must divert to a clear airfield if the weather at the destination is below minima. To summarize the three cases discussed, the various cases areset out in the table: — Type of automatic landing Unscheduled Part-scheduled Scheduled Airfield conditions Destination Clear Bad Bad Alternate Clear Bad Committal Point Break-off height Diversion height Take-off Critical Time 30sec 30mm 5hr The degree of redundancy required of an autopilot system foreach application can be expressed by means of a redundancy
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