MB Mk5 ejection seat questions

Hello,
I’ve managed to reset the BTRU. The descriptions above helped me greatly although a major difference was that I had to push in the small plunger first and then the second. Without the input of this forum I wouldn’t have thought it possible or even known that I needed to do this to release the other parts of the seat, thanks.
Paul

Hi Anon and others.
Thanks again for all the terrific info I’m receiving. I’ve removed the BTRU and repainted a few bits that weren’t in great shape, I’m just trying to work out a way to reset it with the tools I have but am sure it won’t take long, it is a pain though that when you push the long plunger in the rod / sear for the scissor shackle release comes out. Most of the seat functionality has been restored now except for the safety lock for the faceblind. Here’s my progress so far.
Paul

Here is a pic I took today of my Mk.5 parachute pack on my Phantom seat, Ozjag. I presume it will be the same or similar on your seat.

Also a pic of the (Phantom) Mk.7 RSSK PSP and seat cushion which, I would have thought, were the same for Mohawk. These are also on my Phantom (H7) seat but fit the Mk.5 seat bucket OK.

Also, the Koch connectors, which are on long parachute risers. Note that each strap has an angled shackle, and these fit flat against each other and interlock so, effectively, becoming a single shackle for attaching to the retraction web belt but dividing when the chute is deployed.

Anon.

Unfortunately the system isn’t allowing me to upload the files due to some “error” or other. If you PM me with your e-mail, Ozjag, I’ll send them across that way.

A.

I am amazed at how much I have learnt just from this thread. With all of the detailed descriptions above I am really starting to understand where everything goes and how it all depends on it all to be in place so that it can work properly. The finished seat will be going in my Mohawk cockpit but that is a project that is in storage for now, I don’t expect it to be completed for a number of years.

Anon – the Mohawk uses a harness similar to a MA-2 but slightly different. Early seats used RJE fasteners and later seats Kochs, mine was used in a transitional period where both were used so I’ll probably go with whatever type I see first. The Kochs are fitted with the male portion on the harness and female on the seat similar to US Navy systems, I understand that USAF is the opposite. There is also anecdotal evidence on the net that some Mohawks in Vietnam actually used chutes and harnesses from F-8 Crusaders as the supply chain was better for them than through the Army system.

I’ve attached a photo of a small part that came with the seat, it’s just a thin tube about 10cm long, does anyone know where it goes?

The BTRU on my seat is set to 5000m and I have 3 trip rods for it as well as 2 trip rods for the drogue gun, on this seat the trip rods are connected to a crossbeam attached to the main gun rather than the cockpit wall, unfortunately I’m missing the main gun but will be able to make a dummy up eventually.

Parts that I do have are; headrest, parachute restraining straps (2 sets), trip rods, drogue shackle, drogue slug shackle (x2), oxy bottle and leg restraint lines (the Mohawk uses 1 blue and 1 white one)

Parts that I don’t have are; drogue chute, drogue link line, drogue withdrawal line, main parachute, main parachute soft case, PSP (or RSSK as the Americans call it), seat cushion, leg restraint garters.

Regards Paul

PM Sent

Terry

A quick question for you, Ozjag: does, or should, your seat have Koch connectors for its pilot harness? Mk.5’s in US and UK Phantoms used the Koch with the crew wearing the MA-2 integrated torso harness.

Anon.

As XF940 says, nothing will hold in place until the BTRU is reset.

The BTRU or Time Release or Barostat is just a loaded spring, which is “fired” by pulling out the stainless steel “bullet” which is attached to the trip rod, which attaches to the aircraft structure behind and alongside the seat. The trip rod and “bullet” stay with the aircraft upon ejection. The only instance when the BTRU will not “fire” is when the seat/BTRU is above the safety altltude determined by the circular baro unit screwed into the BTRU. It says on this unit whether the BTRU will fire at 10,000ft (UK seats) or 13,000ft (US seats). However, at ground level this is of no matter, the unit will fire when the “bullet” is removed.

BTRU’s seem to have a bit of an ill-deserved reputation. They are only, as stated, a loaded spring when set but will give you a fright if/when they are triggered. They are relatively easy to reset, if you have the patience, a vice and a clamp – as well as the “bullet” interlock to insert when the unit is cocked. The longest plunger is the timer spring and this needs to be pressed in all the way, which it does so with a whirring sound, so a large vice or a cramp-type clamp can be used to do this. I have also done it by just pushing down on the unit. However, once this long plunger is fully in then the shorter plunger needs to be pushed in. This is spring loaded too and is the bit that “fires” when the BTRU “bullet” is pulled and the timer spring/plunger has wound down (with a loud clockwork whirring sound). This short plunger can be pushed in (again, against quite a powerful spring) and it will then click when fully home and stay there – as long as the large plunger is still compressed. At this point the “bullet” should be inserted in the bottom of the unit and this will hold everything in position. When releasing the long plunger clamp the unit will unwind a few whirring times but then stay there ready to be triggered. And that’s it. Once mounted to the seat (if it isn’t already) then the crewman harness can be locked into position.

It can be a tricky procedure to lock in the various harness release points but once done and some familiarity is gained it is fairly easy. Assuming one has not got the special MB tool for locking in the top harness/para pack restraints then a long screwdriver will do. As XF940 has said, make sure the green harness restraint belt in the back of the horseshoe tunnel comes straight out ABOVE the two swing lugs. Pull it out a good way as it will try and retract and is much harder to do the rest of the fit. The two para pack restraint straps come down from the sides of the head box, over the para pack and then each is fed into the tunnel, each strap having a ring on the end and each ring being placed over their respective swing arm. The swing arms and rings are then folded backwards/inwards so that the outer ends of the swing arms are almost touching – except that they will be either side of a slot, which is in the aluminium body of the upper harness lock at the back of the tunnel. It is into this slot that the single metal loop on the end of the green restraint belt is placed, after first feeding this green belt downwards through the black shackle on the upper rear of the crew harness and back in to the tunnel. It will go in some way but will need to be pushed all the way in with a long thin screwdriver resulting, hopefully, in a loud, positive and reassuring click. Job done.

The para pack restraints are now firmly held on the swing arms and these won’t swing out because they come up against the green belt’s metal loop fitting locked in the upper harness lock aluminium body. Yes, it sounds complicated but it isn’t really, it’s just difficult to put into words for someone to understand without training or terminology. It is exactly the same for the H7 (Phantom) seat so if you can get hold of a copy of the seat manual for that you will have all the info you need. The lower harness fittings push into the seat pan either side of the PSP and, again, you will need your special MB tool or a long screwdriver to click these home. These lower lugs should first be fed through loops on a Y-strap (on the Phantom seat), which goes under the PSP and up through a tunnel at the front of the seat bucket to the neg “g” strap/harness quick release fitting (QRF) into which the four lugs of the crewman’s harness are locked.

If you need any further assistance, post or PM me. I may be able to get send some pictures to assist.

Anon.

Hi Paul,

The first photo you show is exactly where the straps go. The lean-forward inertia strap goes ABOVE the swing-lugs, through the harness shackle and then loops under. You should see a vertical slot in the gubbins with the lean-forward mech, and there is a pin that goes through the ring on the end of the lean-forward strap inside that slot. But as I say, without getting the BTRU reset and having all the bits in place nothing will stay put.

You will likely also find a set of slots in the rear of the seat pan that the lower harness strap lugs also lock into – think I can just make it out in the rear RH side of the second photo of the seat pan.

Not sure about the chute harness – the American STENCEL seats had a separate harness but I believed that the MB seats had a seat-attached harness, and the lean-forward stuff and the fact that the seat uses a standard horseshoe pack would indicate that is the case. Only sure way would be to see a fitted seat.

Good luck with the restoration. I know just how hard it is to get the correct parts, and most of my seats were restored well over 10 years ago now when soft kit was much easier to acquire – I’d hate to have to do it now. Feel free to contact me when you get close to putting it all back on and I’ll see if I can help further. Resetting the BTRU without the correct tool can be a bit ‘traumatic’ but it’s possible, you just have to be very careful. I always did it with the BTRU off the seat and fitted it back ‘live’ (carefully!).

Mark.

Hi XF940
Thanks again, that explains it nicely. I was wondering what those little swing arm things were for. It looks like I am a long way from a) getting all of the necessary parts and b) getting everything assembled but may need to PM you in the future for some extra help especially with resetting everything, currently the BTRU has been released to allow the scissors to open.
Here are a couple of very quick photos, the seat appears to be similar to most other MB seats in general arrangement although I believe 1 major difference is that the pilots wore a special harness which attached to the parachute when they got in rather than the parachute harness being attached to them (could be wrong on that though).
Theres so much to learn and I am always grateful for the assistance of those who know what they’re doing.
Cheers Paul

Not familiar with that exact seat, but most MB seats of that era are very similar.

The straps go unto the recess where the lean-forward reel is located. There are two ‘swing-arm’ lugs that the rings on the end of the wedge-pad straps go over and these are then located back into the recess. The ring on the ‘lean forward’ reel that goes through the shackle on the harness loops back and goes in between and is retained by the locking lug in the lean-forward mech, retaining the two swing-lugs. When the seat separation mech is reset, these are then latched into place. Until the complete seat separation mech is reset these will not stay in place and will pull away – it is what is supposed to happen to allow the chute pack to fall free from the seat.

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The image is from a 3KS1 Vulcan seat but as a rule these bits work in the same way until the newer generation of seats came along. However, it may not be the same as yours as I don’t know the seat but feel free to post a photo…..

It is almost impossible to get the seat ‘together’ until you have all the parts that then need to be built up in the correct order to be able to reset and latch up the seat. A lot of people are concerned about resetting or fitting a reset BTRU unit because of the potential energy it imparts into the seat with the possibility of causing injury, but unless you do fit everything in and reset the BTRU, the seat harness and chute won’t lock in place.

Hope this helps.

Thanks, its a J5B from a Grumman Mohawk so I’m not holding my breath to find all of the correct bits and may have to ‘make do’ with some items. I’ve got 1 question at the moment, the straps that come down from the headbox and hold the main parachute in place, how are they attached at the backrest end?
Cheers Paul

That’s pretty much it. But depending on the seat, the manual separation sequence will be different as there needs to be a mechanism to disconnect the chute pack from the seat should the automatics fail (drogue gun, barostat etc).

Having rebuilt 5 seats in the past one of the hardest parts is finding the correct bits for the particular variant of seat as Martin Baker were masters at ensuring non-compatibility between seat marks of the same type.

Good luck.