BAC to proceed with the development of the 500 series, a more powerful, stretched version of the original One-Eleven 200 launched by BEA's independent rival British United Airways in 1961; its second repeat-order for Tridents launched the 3B series, a more powerful, aerodynamically enhanced, stretched version of the earlier Trident models already in service with/ordered by the airline. The 3B had an unusual (and noisy) feature, a small fourth engine (a Rolls-Royce RB162) in the base of the tailfin to increase power during takeoff. The One-Eleven 510EDs ordered by BEA had a range of and were configured for 97 passengers in a single class while the Trident 3Bs entered service with the airline either in a 152-seat, single-class or a 130-seat, two-class configuration. BEA's BAC One-Eleven 510EDs and Hawker Siddeley Trident 3Bs also featured common instrumentation to attain a high degree of commonality. This was achieved by having Smiths supply the avionics of both aircraft, which in the case of the One-Eleven replaced the Collins avionics found on all other 500 series. The differences in flight deck layout between the 510ED and other 500 series were so significant that a separate aircraft type rating was required to fly the former. Another notable difference between the 510ED and all other 500 series was that the former's Rolls-Royce RB163-25 Mk 512-11 Spey engines lacked the latter's Mk 512-14DWs' demineralised water injection system, and that these engines were rated at the same thrust as the Speys powering BEA's Trident 2Es and 3Bs. To compensate for the 510ED's lower rated engines, no forward integral airstairs were fitted. The resulting weight saving permitted an increase in payload. It also offset (to some extent) the additional costs of customisation to attain commonality with the Trident 3B. Although unique in the context of the 500 series, the absence of forward airstairs was a characteristic BEA's 510EDs shared with BUA's 201ACs. On 4 January 1966, BEA's biggest independent rival, BUA, simultaneously launched daily BAC One-Eleven jet services from Gatwick to Glasgow, Edinburgh and Belfast, which indirectly competed with the corporation's London–Scotland and London – Northern Ireland trunk routes from Heathrow. This allowed BUA to steal a march on BEA by becoming the first scheduled all-jet operator on UK domestic trunk routes. In November 1966, BEA increased its shareholding in BKS Air Transport to 50%. In March 1967, BEA established British Air Services as a new holding company for