Helicopter DAFCS Problems?: Ask the right questions first

What to ask the flight crew when problems arise.

Since certification in 1983, Honeywell's Digital Automatic Flight Control Systems (DAFCS) for helicopters have become the standard against which all other systems are judged. The number of variations installed in over a thousand Bell, Eurocopter, Sikorsky, and Mil/Kazan helicopters is evidence of this.

In addition to their four-axis capability, these systems also introduced built-in-test (BIT) and self-diagnostics never before seen in a helicopter AFCS. But what do you do when the system can't tell you what's wrong? Having some basic knowledge about these sophisticated systems and some idea of what to ask the flight crew, or yourself when problems arise can save many hours of troubleshooting and thousands of dollars wasted on replacing the wrong component.

The SPZ ("Spee-zee") Series

Prior to Honeywell acquiring Sperry in 1986, the SPZ-7000 was the first Sperry system to provide full four-axis flight control for helicopters, i.e. pitch, roll, yaw, and collective. The dual system is found on many versions of the Sikorsky S-76 as well as the Eurocopter 365N and N2, while a simplex version is found on the Bell 222U and 230. A three-axis (pitch, roll, and yaw) version, dubbed SPZ-7100, was developed for the BK-117. In 1991, the SPZ-7600 was introduced which improved on the SPZ-7000 and added optional search and rescue (SAR) modes including Mark-On-Target approach to hover. The SAR option was first certified on the S-76 and was followed by the Bell 412 in 1992. An anti-submarine warfare (ASW) version can be found on some Bell 412s.

While all of the previously mentioned systems provide fully coupled automatic flight control, approved for single pilot IFR (SPIFR), there are uncoupled, "flight director only" versions installed in the Sikorsky S-70 and the Mi-17KF. As stated earlier, self-diagnostics are incorporated in these systems to help identify and isolate the causes of malfunctions. Three types of BIT detect specific faults at system power-up (power-up test), after power-up (continuous test and monitoring) and preflight test. Errors detected by these routines are displayed on the instrument panel and should be noted by the flight crew. Power-up and continuous tests check basic system integrity and operate independent of pilot action. Prior to takeoff, the pilot uses the preflight test to verify proper operation of peripheral sensors and controls.

Though error codes can be extremely helpful when troubleshooting system problems, there are still plenty of times when a code simply isn't displayed, usually because one doesn't exist that correlates to what the pilot reported. Sometimes, even with error codes the troubleshooting can still be confusing. When this is the case, communication between the maintenance staff and the flight crew becomes critical if the problem is to be solved. As is usually the case, the mechanic isn't present when the problem occurs. So when there's no error code, flag, annunciator, or other direct evidence of what's wrong you'll have to ask the pilot the right questions to efficiently troubleshoot and resolve the issue. Here are just a few examples.

Is it an autopilot problem or a flight director problem?

Many mechanics and pilots don't understand the difference between an autopilot (AP) and a flight director (FD). Frequently the entire system is referred to as the "autopilot" even though, in classical terms, autopilots and flight directors have totally different roles. Knowing the simple difference between the autopilot and flight director will help you focus your attention in the right area first.

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