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1970's Radio Altimeter issues (Buccaneer)

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1970's Radio Altimeter issues (Buccaneer)

Old 12th May 2019, 13:13
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1970's Radio Altimeter issues (Buccaneer)

I am researching the accident involving Buccaneer XV351 in 1974, and I hope some members might be able to assist with any recollections of abnormal radio altimeter behaviour in-flight. I believe the rad-alt fitted was the Mk 7 B.
At the time, XV351 was a Royal Navy aircraft, flying a laydown attack profile on Wainfleet Range. It struck the water momentarily in a shallow dive approximately 1.5 nautical miles after passing the finals marker. The flight had appeared very normal up to that point - from inside the aircraft, and witnesses on the ground.
The RN AIU found that the AC and DC supply fuses to the rad-alt had blown under excessive current; there is evidence that occurred very shortly before impact. .
The transmitter/receiver equipment was too badly damaged to assess its serviceability at the time.
Publicly available Information on the nature and presentation of rad-alt faults is very limited. I would greatly appreciate a bigger picture drawn from operational experience. Any recollection of pilots' immediate actions to test and rectify problems would also be beneficial.
Many thanks.
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Old 13th May 2019, 16:46
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I don't think this will answer your question, but it might give you an insight: Lt Stephen Kershaw

One of the issues that we had with the Buccaneer Rad Alt was that it was often unreliable below 50 feet and it would, more often than not, lock out at or below that height. Consequently, there was no method of knowing exactly how low you were or, worse, no further warnings if you were in a shallow descent towards terra firma. Unfortunately, and I am not saying this was the case in this accident, far too many aircrew were lost (in many more aircraft than just the Buccaneer) because they were concentrating too hard on other things, like finding and dropping bombs on the target, to be fully aware that the hard stuff was getting closer (before it was too late).

Very sad, I was on 12 Sqn at the time of the accident and it shook us all to the core.

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Old 13th May 2019, 16:46
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t7a
 
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In just under 2500 hrs on the Bucc I never saw a single rad alt fault. There was the occasional blown bulb in a rad alt limit light but that's all.
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Old 14th May 2019, 03:11
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Thank you both - two quite different, but equally valuable responses.
The accident is consistent with pilot distraction insofar as it occurred just after his observer had conned him around the base turn onto the attack heading. Lt Kershaw confirmed that he had acquired the target. The aircraft struck the water a few seconds later. My one reservation concerns time and distance, against height lost.
The aircraft passed over the finals marker at 500 feet descending, according to the observer's altimeter. By my rough calculation, at 420kts, it would have taken around 13 seconds to fly 1.5nm. That sounds like a lot of height to lose, and a long time to be distracted from height monitoring. Assuming a constant rate of descent, the aircraft would have reached 150 feet - release height for laydown - at just over 1nm. The run in to ship target 3 was 9nm; there was no need to be that low so soon.
The electrical evidence suggests there may have been a problem with the rad alt.
A speculative scenario, very much up for discussion: aircraft is flying the downwind leg of the range circuit with rad alt left on the 500ft scale. aircraft passes the finals marker; observer calls the height. Pilot expects the rad alt to start reading on the 500ft scale at any moment, at which point he will begin to use the display needle to control rate of descent to run-in height. This system works time and again. The pilot is focused on the tasks of lining up on the attack heading and acquiring the target. The rad alt display is offering no reading and no 'Off' flag. This does not cause great concern, because there is confidence that it will read imminently. The pilot is distracted from how long he has allowed the aircraft to continue at the rate of descent from when he completed the final turn. The error may have been partly one of time perception.

Is the electrical system controlling weapon release in close proximity to the any part of the rad alt system? On the previous laydown attack, their 4lb practice bombs had failed to release from a CBLS on the port inner pylon. I wondered if the two events could be connected.
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Old 14th May 2019, 07:46
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Like t7a with 2300 hours on the Banana Jet I never had a Rad Alt problem. Worth remembering that it transmitted vertically downwards so was only accurate wings level. The SOP on a range was to calibrate the pressure altimeter to the Rad Alt so you had an accurate height reference in a turn. The Nav/Observer had to use a chinagraph on a strip indicator!
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Old 14th May 2019, 11:44
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Thank you 45-25-25, at what sort of bank angle would the rad alt lose accuracy, and how great were the errors? I have read that modern systems are still accurate within limits when the aircraft is banked.
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Old 14th May 2019, 14:10
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t7a
 
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I can't remember what profile 809 flew on a laydown attack but they would probably have been at 500kts over the finals marker descending to 200ft to be level as soon as possible. Germany squadrons dropped at 150ft 550kts.
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Old 15th May 2019, 00:02
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t7a, on this sortie, laydown was briefed to fly at 150ft, 420kts.
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Old 16th May 2019, 10:03
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Originally Posted by 45-25-25 View Post
The SOP on a range was to calibrate the pressure altimeter to the Rad Alt so you had an accurate height reference in a turn. The Nav/Observer had to use a chinagraph on a strip indicator!
Just picking up on that point, did pilots adopt the practice of winding the millibar sub-scale on their pressure altimeter to align it to the rad alt reading?
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Old 17th May 2019, 13:23
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We need to be careful here with what was briefed and what was flown. I too have 2500hrs on the Buccaneer and am a QWI. Wainfleet Range, whether the target was 9nm ahead or 3nm ahead has no bearing on what height we should be at. The Buccaneer weapon system was analogue, which meant that there was a degree of lag in the system. The Control and Release Computer was driven by cogs, pulleys and string (believe me, I have seen inside one) - very accurate cogs, pulleys and string, but not as accurate as modern digital systems. As a result, on the Wainfleet Range Laydown pattern, which was flown downwind at 1500ft around Boston Stump, the crew would have been briefed to be at delivery height as soon as they levelled on the target run. The navigator would have been looking in at his radar screen trying to locate Target 3 (The Ship) at Wainfleet, which did not always stand out well on the Blue Parrot when the tide was out (which I think was the case on the day in question). Even in a visual laydown attack the navigator would be looking for the target on his radar to assist the pilot with sighting it. Also, as 45-25-25 states, the height check should have been undertaken at attack speed and height over the sea. The Buccaneer, at the time, had a strip altimeter in the back seat on the left hand side of the cockpit below the level of the navigator's thigh that was fixed to 1013Mb. To gain an accurate height check the navigator had to etch, with a thick chinagraph pencil, the height at which the height check was carried out. Most navigators are right handed, so this was also done below his thigh level with the 'wrong hand' - accuracy? Hmm! Try it. Sit in an armchair with a pencil in your right hand and try etching a line at 90 degrees to your left thigh on a piece of paper that is below your left hand side. Now imagine doing it whilst wearing an immersion suit, a skeletal harness and strapped into a bang seat!! So we used our left hands (easier but not as accurate as the right hand). What’s more, the etched mark was probably about 50ft thick.

Of course, pilots did wind the millibar sub-scale on their pressure altimeter to align it to the rad alt at the same time.

As a QWI, I never heard of anybody releasing practice bombs in Laydown below 200 feet, although I accept that that may have been practised for Strike options in Germany. The Navy were occasionally, however, a law unto themselves - I don't mean that as a criticism, but they were.

In my view, whether the Rad Alt had locked out below 100 feet, was temporarily unavailable because of the bank angle or had failed (or not), it was probably not the sole reason for the loss of the aircraft.

Last edited by Fg Off Bloggs; 17th May 2019 at 16:49.
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Old 17th May 2019, 16:50
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Thanks again, Fg Off Bloggs, Your information regarding the lag time of the analogue CRC is interesting - I had not considered that aspect.
From what I gather, 809 experimented for a short period around that time with laydown at 150ft/420kts to improve accuracy.
It does appear that the Navy was a law unto itself - and I certainly don't mean that as a criticism either!
I tend to agree with your final comment: I am looking at how a technical failure might feature as a contributory cause, but cannot see it as a sole cause.
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Old 17th May 2019, 18:10
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Just to follow up on your question about the weapon system, the Rad Alt and the electrics thereof. I don't think there was any correlation, but a systems man would know better than this mere QWI! I suspect not, though. Unlike the Rad Alt system in the Tornado (which I also flew) I don't ever remember the Buccaneer Rad Alt being linked to a weapons delivery mode, hence the requirement to correlate it with the 2 pressure altimeters, front and back seat, to give an accurate pressure height over the target.

You might learn more (and even more than you bargained for) here:

https://www.pen-and-sword.co.uk/Adve...rdback/p/14012

https://www.amazon.co.uk/Adventures-Cold-War-Fast-Jet-Navigator-ebook/dp/B074SJHVZL/ref=tmm_kin_swatch_0?_encoding=UTF8&qid=1556545351&sr=1-1 https://www.amazon.co.uk/Adventures-Cold-War-Fast-Jet-Navigator-ebook/dp/B074SJHVZL/ref=tmm_kin_swatch_0?_encoding=UTF8&qid=1556545351&sr=1-1



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Old 17th May 2019, 20:48
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And again, I thank you Fg Off Bloggs.
I may well invest in a copy of your recommended reading. It appears to be written by a fellow who knows what he's talking about (!)
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Old 18th May 2019, 09:01
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Fg Off Bloggs wrote:
The Control and Release Computer was driven by cogs, pulleys and string (believe me, I have seen inside one) - very accurate cogs, pulleys and string, but not as accurate as modern digital systems.
Indeed! I likened it to a committee sitting round a table when one of them announces "There's been a request to drop a bomb!" as the pilot squeezes the accept bar. The servos (6 IIRC?) then take a vote and if all agree, out comes the pin and off goes the bomb. Amazing that even a stumbling student such as I was could still get a bomb within 100 ft of the target on a medium toss attack from a little over 3 nm away!

Back to the topic of rad alts - wasn't there an accident in the early '70s where mutual interference between rad alts on similar frequencies was thought to be contributory?

On another website, a 24 Sqn SAAF pilot once wrote the following:
The stillness of the night causes another problem. As the surface of the lake is so calm our radio altimeter emanations are not returned by the water surface of the lake. The beams go straight through the water to the lake bed giving us a false return of our height. As we are doing our runs at two hundred feet this can deceive us as to our actual height, which could be a bit fraught! We do have a setting to alleviate this, but with an attack speed in the order of 480 knots there is not time to play around.
Even in the Vulcan, flying at low level over those dense forests of tall trees in Canada, I wondered quite what was providing the height indication on the rad alt - was it the tree tops or the ground?
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Old 18th May 2019, 10:10
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The rad alt 7b on the Buccaneer was replaced by the 7c, this version had a widget to prevent cross talk between aircraft in formation.
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Old 18th May 2019, 11:13
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Beags, surely it was the nav who put the medium toss bomb within 100 ft. All you had to do was accept and pull!

PS, and not crash in the recovery.
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Old 19th May 2019, 16:51
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It was the nav who found and locked onto the target and set the correct wind and, if necessary, Target Course and Speed settings on the C&RC, but if the pilot did not accept at the correct point in range and then follow the aiming mark accurately with a smooth pull and no bank then the bomb would not end up at 100ft (or closer)! Team concept in the Buccaneer - crew cooperation at its best!

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Old 19th May 2019, 17:30
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Originally Posted by Fg Off Bloggs View Post
It was the nav who found and locked onto the target and set the correct wind and, if necessary, Target Course and Speed settings on the C&RC, but if the pilot did not accept at the correct point in range and then follow the aiming mark accurately with a smooth pull and no bank then the bomb would not end up at 100ft (or closer)! Team concept in the Buccaneer - crew cooperation at its best!

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I know, I was one. QWIs, still no sense of humour!
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Old 19th May 2019, 19:17
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Team concept in the Buccaneer - crew cooperation at its best!
Indeed - taught to us from Day One of the OCU.

After shut down, it was always SOP to check the multitudinous number of seat and canopy pins before leaving the aircraft. One day as I was doing so, my staff navigator just climbed out and walked off. Had he checked his pins? I wasn't certain, so I did so. Sure enough, he'd put a couple of pins in the wrong holes, meaning that the seat wasn't entirely safe. So I swapped them over, then went to catch up with him....
"Where the ***k have you been?"
"Checking your pins - you'd hadn't put them in the right holes"
"B*ll*cks - I know what I'm doing, you're only a student"

Crew cooperation wasn't universal, sadly - do as I say, not as I do!
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