The Vulcan was designed as a ‘safe life’ aircraft, with a well-defined limit on how long it can carry on flying. One airframe (the 60th) was removed from the production line and placed on a specially built test rig at Woodford, this was known as the “Fatigue Test Specimen” (FTS). Using hydraulic jacks attached to various points on the airframe, BAe could simulate different flight profiles and therefore work out how quickly the fatigue life of the fleet would be consumed.
As the Fatigue test went through it’s “life” various defects became apparent. As the defects occurred, repairs were made to the FTS, and then these repairs were used as a basis for repair schemes for use on the in-service fleet, (the FTS always being far ahead of the fleet in terms of the life consumed). Repairs and modifications were programmed in various phases, timed to extend the life of the fleet. The third phase of the programme involved strengthening the front and rear spars and the landing lamp apertures but this work was cancelled due to the introduction of the Tornado. The rear spar modification (Mod 2222) was finally carried out on XH558 as part of the Major service during restoration.
During the FTS life the lower skin of the wing cracked just in front of the main undercarriage bays. Although the structure is very solid a lot of the flexing in the wing is concentrated at this vulnerable point. Initially two small repair plates were bolted over the point where the cracks had appeared. Unfortunately this just chased the cracks further inboard, therefore the plates were replaced with four newer, longer plates (Mod 2081, applied to XH558 in 1969). During the residual strength test the skin cracked again just inboard of the 2081 plates, so the FTS was repaired again, the old plates were removed and new plates were made up which extended the repair approximately 18 inches inboard and also took in an extra two rows of fasteners just aft of the existing plates. No drawings of this Mod were made, though a set of sketches were added to the FTS records.
One of the key issues identified by Earl Pick during the project feasibility investigation of 1997 – 2000 was whether sufficient design data remained available to allow these fatigue life extension modifications to be successfully embodied on XH558. With BAE Systems help, the extant FTS record sketches did provide sufficient technical data to form the basis of the drawings prepared for the Trust by Marshall Aerospace, just as they would have formed the basis for the phase 3 RAF Modification.
The work required involved the removal of the four existing reinforcing plates either side of the aircraft and replacing them with four new plates manufactured from flat sheet. The new plates are the same specification as the old ones but extend inboard approximately 16” further and are about 2” wider.
The plates are secured using approximately 250 screws with nuts and washers on the inside and 130 rivets per side. The screws were replaced by Hi-loc fasteners which do the same job but were much less expensive being available “off the shelf.” Access to about one third of the bolts securing the plates is through the leading edge access panels (a hole about 16” x 9”). Fitters thus had to spend a considerable amount of time inside the leading edge access tunnels. Due to the short timescale, four contractors from JMC Recruitment in Exeter joined the VTST team to help out with the work (having made sure that at least two of them could fit into the access panels!).
The plates were cut to a rough size, then attached to the bottom of the aircraft and the majority of the holes transferred from the aircraft to the plates. Any additional holes required were either transferred from the original plates or calculated using the drawings and the aircraft. The plates were then cut to final size and sent away for the surface finish to be applied, before finally being riveted back onto the aircraft and painted over.
A second Mod was required around the landing lamp apertures. The FTS cracked at the forward inboard corner and the aft outboard corner, which gave rise to a fleet check in 1981 during which several of the in service airframes (including 558) were found to be cracked. This created the need to inspect the apertures at regular intervals. The FTS was repaired and the drawing that was produced would have formed the basis for the Mod which was to have been embodied as part of the phase 3 program.
The modification involved replacement of two 5’ sections of stringers at the forward and aft edges of the landing lamp apertures. To remove the old sections of stringer the existing skin was cut approximately 2’ inboard of the aperture, and replaced with a new section of skin, manufactured from sheet and attached to the old skin with 9 joint plates per side. The outboard skin plate also had to be removed out to an existing skin joint and replaced with a new section.
There are approximately 55 packers, doubling straps and joint straps etc. per side, mostly flat plate but all had to be cut from sheet and drilled, with approximately 2,000 rivet/bolt holes per side. The area was paint stripped so all the rivets which needed to be removed could be easily seen and cut lines marked accurately on the skins.
With the skins removed, cut lines were marked out on stringer 1 and 5 in accordance with the repair drawings. Then the stringers were cut, the rivets and bolts attaching the stringers to the rib butterfly brackets removed, and finally the cut stringers were taken out. New parts were made, painted and wet assembled before final installation – the intention of the Mod being to spread the load more evenly around the aperture and hopefully prevent any further cracking.