Older airplanes - PARALLEL Y/D (Letter P is first on the alphabet) -
Newer airplanes - SERIES Y/D (Letter S later than letter P in alphabet) -
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747s all have 2 SERIES Y/D, and had them since day one..
A 747
flies perfectly well with both Y/D failed... NO RESTRICTIONS.
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Older Boeings -
The old 707s had PARALLEL Y/D. The switch on the pedestal had 3 positions, which were OFF, Y/D and A/P. Takeoff and landing was flown without Y/D engaged. Flying the 707 without Y/D engaged was a guarantee to have airsick passengers in the tail... Later 707s got one SERIES Y/D... I threw my old 707 manuals away, sorry, cannot recall more details.
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For the 727, both 100/200s, all have SERIES Y/D, 2 of them. In case of Y/D failure, 727 is very unstable plane. Altitude limitations (cannot recall) are some FL 270 with failed Y/D.
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Happy contrails