I have mentioned a few times on the site that the Vulcan NBS system was an analogue computing system, as distinct from todays digital systems. The height chain was no exception.

Internal Aids Approach

The following is a brief ‘skip’ through the height chain, using my NBS course notes as my source.  The majority of the Height Chain is contained within the Calc 3, which is located behind the Nav Radar’s seat in the pressure cabin.  Reading my notes I have to admit that some of the detail is a little slow in coming back to me.  The intricacies of the ‘X and Y relays’, the ‘non-linear linkage’ and the implications of the ‘PC3 test facility’ seem to have faded a little.  I hope that if I make any mistakes, some of you will get back to me.

In the sections below remember that because this is an analogue system, so there are references to voltage analogues, static pressures etc; these are analogue inputs as distinct from digital inputs.

For the uninitiated; ‘why do we need a height chain’?  Simple trigonometry shows that if the aircraft is on the ground then the range to target from the aircraft (plan range) is the horizontal distance from the aircraft to the target.  However, if the aircraft is above the ground then there is a triangle and the distance from aircraft to target (slant range) will be greater than when the aircraft was on the ground. The NBS requires the height information for the following chains;

a) The Display Chain where the height voltage delays the start of the timebase and shapes the linear timebase rundown.

b) The Marker Chain where the height voltage is fed to the triangle solver to produce the Rs marker.

c) The Ballistic Chain where a height shaft setting is required for the solution of the forward throw formula.

The primary height system in NBS employs common barometric pressure, but this is calibrated to ICAN settings, the system has to be adjusted to local settings; this means that we will need a process for height correction .  It was mentioned above that there will be a difference between the plan range and the slant range, and this difference will vary with the aircraft height. However, as the target is unlikely to be at sea level, we will also need to take account of target height. The higher the target the smaller the effective difference between plan and slant range. The Nav Radar will need to input target height into the height chain for correct bomb forward throw calculations.

The diagram below shows three height knobs on the left, one on the Calc3 and one on the 585 are for target height input, with a height correction knob also on the 585. There is also the static pressure input to the aneroid capsule.  Setting target height on the 585 will feed a correction to the height differential and also to the height drum on the Calc3, however, setting target height on the Calc3 will not feed through to the target height dial on the 585.

With the system functioning normally a change of aircraft height causes a change in the static pressure fed to the aneroid capsule, this will move the ‘slug’ so causing an error signal to be induced in the moveable coil assembly, that causes the differential to return the coil to the point where there is no error signal.  The net result is that the Calc 3 height counters are driven to the new aircraft height. Because pressure change is not a direct linear relationship with height, there is a need for the ‘non linear linkage’ to act as an analogue equivalent to the relationship between height and pressure difference. This means that because the pressure difference between ground level and 10,000ft is not the same as that between 40,000ft and 50,000ft, the movement of the coil will be different, and this will be determined by the non linear linkage.  The left hand side of the diagram below takes the feeds from the right hand side of the diagram above.  With the Calc 3 height cyclometers at the correct aircraft height, and having done a height correction (bit on this later) the outputs are fed off to the various waveform generators so that the NBS radar is fed with correct markers.

In the top diagram there are two knobs on the bottom left, one is the Target Height knob on the CU585, the other is the Height Correction knob on the CU585. Because of pressure differences around the globe there is a need to use altitude correction (alticor) settings.  One of the tasks for the Nav Radar is to choose a ‘height finding area’ (HFA) on the planned route where he will be able to carryout an exercise to obtain an alticor.  Usually the HFA will be an area of sea where the aircraft is at a steady height. Using the CU585 height finding circuitry the Nav Radar can use the height correction knob to adjust the radar so that he has a coincidence between the ‘first ground return’ from the radar and the NBC height strobe; this is done by a ‘blinking’ light circuit on the 585.  See diagram below.  When the height finding exercise is completed the target height drum on the Calc 3 will indicate the alticor found in the HFA, a comparison can then be made between this true alticor and the planned alticor given to him in the met briefing.  This alticor correction can then be used on any planned alticor that he has for the target area, so ensuring that an accurate target height will be set for the bombing run giving correct forward throw and generating correct input to the marker and display chains.

If anyone wants a memeory test, I have a copy of the 12 Test Questions that came at the end of the lectures on the height chain.  Not sure how many I would get right now.

I have attached some further comments from Roy Brocklebank regarding Height Finding.

The height on the Calc 3 was fed automatically from the barometrics and, with the height of the target set on the CU585, would indicate the height above target, but this would need to be corrected for barometric pressure.

The Met Forecast would include a forecast ‘d-factor’ which was the forecast difference between actual and standard altimeter height readings. With the d-factor set on the Calc 3 the Calc 3 would indicate true height. It was SOP to check the Calc 3 height either against the Alt 6 or through an H2S Height finding technique. Ideally this would be done over the sea.

The radar tilt was set at maximum down and the height control on 585 adjusted until ‘neon first flicker’. This would then have set the Calc 3 height to true height. It was then cross checked against the Alt 6. If the height system had failed then it was possible to set the height directly on the Calc 3 using PC (Primary Check) Set H (Height). This would run the height carriage up or down at 3000 feet per minute. Clearly if it had stuck at 17200 feet then you needed a good 10-12 minutes to run it up to a high level bombing height near 47000 feet or higher.

It was normal to do a height check before each attack if attacking in different parts of the country.

For high level attacks deep in Russia the last over water height finding might be over the channel or North Sea. To allow for height finding over land special ULAC were produced. There were Lambert’s charts at a scale of 1:2188000 with ‘uniform level areas’ marked on them together with the height of the level terrain there in. To use a ULAC you would set the terrain height on the target height dial and then conduct the height find as normal.

Internal Aids Approach

The Internal Aids approach was a means of talking down the pilot to a break-off height, using the H2S radar as the talk-down tool.  I used to like doing these, but I needed the OCU notes to remember how they were done.  The break off height was officially 600ft, but if the conditions were good then I have done them down to 300ft, provided the line-up was good and the pilots had good visual contact with the lights. The idea was pretty simple, basically it involves using radar settings to ‘frig’ range markers and heading indicator. The true runway heading is a known, so a heading marker can be set up on the PPI with the 585 set in PC and EMERGENCY mode. The radar height dial can be set so that it produces a range marker at defined ranges. You now have a range and bearing marker.

In the diagram below the aircraft can fly to the overhead, then go out on the downwind leg to a 10 mile point, at a height allocated by ATC; at this point the pilot turns ‘base’ and descends at 2100ft AGL.  With the range marker on the PPI, and the airfield also showing and the 1/4mil. scale the Nav Radar then turns the aircraft to runway heading (allowing for a/c turning circle) so that the pilots will ideally roll out with the heading marker straight down the runway. The Radar will call heading adjustments to the pilot to keep the runway tracking down the heading marker as the aircraft approaches the airfield.  At this time the pilot maintains 2100ft.

Using the table at the bottom of this page the range marker can be set at desired range points; with 42,000ft on the emergency height pot he has a 7 mile marker. When the touchdown point hits the range ring the Radar calls for a 300ft/mile descent rate. From this point on the Radar monitors the tracking of the runway along the heading marker to ensure he stays on runway centre line, at the same time the Plotter cranks in the new emergency height settings each time a range point is hit.  So, having started at 7 miles, the plotter sets up the 6 mile marker while the Radar monitors runway heading. Once the six mile marker is hit he tells the pilot the range and that he should now be 1,800ft AGL. The pilot maintains correct rate of descent for each mile mark called while the Radar monitors heading. When the 2 mile marker is hit the aircraft should be 600ft AGL and if all has gone to plan the pilot is at break off with the runway on the nose. He may then elect to continue the talk down to 300ft if he’s comfortable.  But not officially.  Done well they are a good crew exercise, and who knows when it might be needed in an emergency.

Emergency Height set (ft) Range in miles Aircraft height above touchdown 42,000 7 2,100 36,000 6 1,800 30,000 5 1,500 24,000 4 1,200 18,000 3 900 12,000 2 600 6,200 1 300