Boeing 737 - Service Difficulty Alert

AL 1998-02
17 August 1998

Flight Spoiler Power Control Unit

During climb-out of a Boeing 737-210C aircraft, the B hydraulic system failed due to loss of fluid. The failure also resulted in the No. 7 spoiler floating up and the flightcrew had to use ailerons, rudder and differential power to control the roll for the duration of the approach. 

The loss of hydraulic fluid resulted from the circumferential cracking of the aluminum cap (P/N 69-35541-2) on the thermal relief valve of the No. 7 flight spoiler power control unit (PCU).

Cracking of these caps is not a new problem. The problem was recognized by Boeing and a steel cap was introduced in 1977. The problem was also addressed in two Boeing In-Service Activity Reports (ISAR), 83-02-2764-10 and 91-11-2764-10. The first ISAR had recommended replacement on an attrition basis; in the most recent ISAR, Boeing recommended that operators consider replacing the aluminum caps (P/N 69-35541-1 and -2) with steel caps (P/N 69-35541-3) at an earlier convenient maintenance opportunity.

Apparently not all Canadian operators are aware of the Boeing recommendation on the replacement of the aluminum caps. Although the B737-300 thru -500 were manufactured with the steel caps, the aluminum and steel caps are interchangeable and it is possible for the newer aircraft to have the older caps installed during maintenance.

Transport Canada recommends that at the next convenient maintenance opportunity, operators of Boeing 737-100 thru -500 aircraft accomplish the following:

  1. Replace any aluminum caps (P/N 69-35541-1 or 69-35541-2) with steel caps (P/N 69-35541-3) fitted to the thermal relief valve of the flight spoiler actuators; and
  2. Purge any spares holdings of aluminum caps (P/N 69-35541-1 or 69-35541-2).

For further information contact a Transport Canada Centre, or Mr. Ian McLellan, Continuing Airworthiness, Ottawa, telephone (613) 952-4362, facsimile (613) 996-9178 or e-mail

For Director, National Aircraft Certification

M.J. Martin
Acting Chief, Continuing Airworthiness