I3001 Discuss Items

Question 1

Attitude Instrument Trim/Flight/Scan

Answer 1

Attitude instrument flight is that condition wherein the pilot controls the aircraft with reference only to the aircraft instruments.

The pilot must be able to rapidly adjust to a basic instrument scan and fly his aircraft by reference to the aircraft instruments in the event he encounters IMC and is unable to maintain suitable visual references.

Amplification and Technique:
While the techniques used for attitude instrument flight in helicopters are essentially the same as those used in airplanes, the helicopter pilot who has previous experience in airplanes should bear these in mind:

1. In helicopters, with either semi-rigid or fully articulated rotor systems, the fuselage is permitted to move independently of the rotor disc. While gravitational, centrifugal and aerodynamic forces tend to align the fuselage in an attitude corresponding to a given rotor attitude, these two attitudes are not in a fixed, corresponding relationship as are wing attitude and fuselage attitude in an airplane. For this reason, the pilot is unlikely to achieve desired results by merely selecting a fuselage attitude and power setting.
2. There is no instrument available directly measuring rotor attitude. Therefore, the pilot must depend more on instruments, such as the airspeed indicator, turn and slip indicator and vertical speed indicator.
3. The Vertical Gyro Indicator, better known as the attitude indicator, is used for “ball park” estimates of rotor attitude. All transitions from one attitude to another should normally be made by the combined use of attitude indicator, turn needle and ball to ensure a “ball park” attitude in balanced flight-then resume an efficient instrument scan.
4. Power changes result in changes in rotor attitude, fuselage attitude, and the relationship between the two. Increasing collective causes the nose to pitch up, roll right, and yaw right. Decreasing collective pitch has the opposite effect; hence, immediately following power changes, the airspeed indicator takes on added importance. Because of the design characteristics of the TH-57, these attitude changes are relatively small when compared with other helicopters.
5. Throughout this manual, when the term “attitude” is specified in terms of directly measurable quantities, then the term applies to fuselage attitude. However, when attitudes are specified in terms of airspeed or rate of turn, the rotor attitude is of primary concern.

Trim and Scan:

The TH-57C is equipped with a force trim system and a 3 axis stability system, “MINISTAB”. For large attitude changes, depress the force trim/maneuver button, set the desired attitude, release the button, and then hold the attitude steady for one second. The MINISTAB will now hold the new attitude, however, in instrument flying, many corrections are very small. For small corrections, working against the trim may be desirable. Depressing the force trim button first defeats the purpose of the force gradient springs to provide stability for small corrections. By working against the trim, there will be some resistance to work against to reduce the tendency to over control. This is called the “fly through feature” of the MINISTAB system. Also, by working against the trim, you only change the MINISTAB attitude retention in the axis of movement while retaining the attitude retention of the unaffected axis.