REPLACEMENT AIRCRAFT STUDIES were conducted throughout the 1970s for the 707/DC-8/and 727-100 fleets which are expected to be largely replaced in the 1980s due to environmental regulations, rising fuel prices and other economic considerations. P&WA chose to offer a new, high-bypass-ratio, high-performance engine, the JT10D, designed at the outset to use new technology materials, processes and components plus the experience of first and second generation turbofans. Late in 1977, P&WA redirected the JT10D program toward a common core engine family to cover the thrust range from 25,000 to 35,000 pounds thrust to satisfy various new airplane requirements.