ST706SO-D
Installation of Skydrol 500C hydraulic fluid. (720-023B) (747-121) (707-121B) (727-21) (707-321). Held by Pan American World Airways INC, which holds 46 approved modifications in all. Last revised September 7, 1978. System: heating, ice protection and utility systems.
What it applies to
- Kind of product
- Aircraft
- Design holder
- The Boeing Company; The Boeing Company; The Boeing Company; The Boeing Company; The Boeing Company
- Models
- 707, 707, 720, 747, 727
- Type certificates
- 4A21, 4A26, 4A28, A20WE, A3WE
- On this site
- Boeing 707 9 registered
Boeing 727 37 registered
Boeing 747 192 registered - The FAA's record
- On the FAA's site
What an approved modification is
A supplemental type certificate is the FAA's approval of a change to a certified design: a new engine or propeller, avionics, an autopilot, floats, a parachute, a larger fuel tank, an ice-protection kit. The company holding it may sell the kit and the paperwork to owners of the listed models; the modification then becomes part of that aircraft's record. The FAA's list says what was approved and for which models, not how many aircraft carry it.
Source: FAA supplemental type certificates (Dynamic Regulatory System). Register counts from the FAA register of October 21, 2026.