Yaw DampersNavigation — Instrumentation — DGCA CPL practice questions

All 6 questions — Yaw Dampers

Navigation — Instrumentation · DGCA CPL. The correct option is marked on each.

1. Q1. Dutch Roll tendency is worst in swept-wing aircraft because they have:

   • A.Low dihedral effect and high directional stability
   • B.High dihedral effect and low directional stability✓
   • C.Low dihedral effect and low directional stability
   • D.High dihedral effect and high directional stability

   Why: Swept wings produce high dihedral effect (strong roll response to sideslip), while providing relatively little keel area for directional damping (low directional stability). This imbalance — strong lateral restoring force with weak directional damping — is the root cause of Dutch Roll. See Section 1 . — Think of Dutch Roll as the result of the two stability modes "fighting" each other: lateral stability (dihedral) tries to roll the wings level, directional stability (fin) tries to point the nose straight — but with swept wings the dihedral effect wins temporarily before the fin can act, cau…

2. Q2. The purpose of the Dutch Roll filter in the yaw damper system is to:

   • A.Increase rudder deflection during turns
   • B.Pass only the Dutch Roll frequency and block steady yaw rates✓
   • C.Reduce rudder authority at high altitude
   • D.Prevent the rudder from returning to neutral after deflection

   Why: The Dutch Roll filter is a narrow band-pass filter tuned to the Dutch Roll frequency. During a constant-rate turn, the yaw rate is steady (DC signal) and the filter output is zero — so the yaw damper does not apply rudder during intentional turns. It only passes the oscillatory yaw rate signal characteristic of Dutch Roll. See Section 6 . — "Band-pass" = passes frequencies in a band; "wash-out" = washes out the DC (steady-state) component. Both terms describe the same function here.

3. Q3. The phase advance circuit in the yaw damper is designed to apply the corrective rudder deflection:

   • A.At the point of maximum yaw displacement
   • B.After the Dutch Roll has completed one full cycle
   • C.At the point of maximum yaw rate✓
   • D.When the aircraft rolls past wings level

   Why: The phase advance circuit shifts the corrective signal forward in phase so that the rudder deflection is applied when yaw rate is maximum. Applying the correction at peak yaw rate maximises the damping force (aerodynamic force is proportional to speed of rudder movement relative to the air). See Section 5 . — In oscillatory systems, maximum damping is achieved when the corrective force is in phase with the velocity (rate), not the displacement. Phase advance ensures this timing is correct despite system delays.

4. Q4. If one yaw damper system fails on an aircraft with a split rudder (each panel driven by a separate system), the remaining Dutch Roll protection is:

   • A.Zero — full loss of Dutch Roll protection
   • B.Approximately 75%
   • C.Approximately 50%✓
   • D.100% — no reduction in protection

   Why: With a split rudder, one yaw damper system controls the upper rudder panel and another controls the lower panel. If one system fails, only half the rudder span is under yaw damper control → approximately half the damping authority is available. See Section 7 . — Split rudder is specifically designed for this failure case — it avoids total loss of yaw damping while keeping each half independent for maintenance and safety.

5. Q5. Why does aerodynamic damping of Dutch Roll decrease at high altitude?

   • A.Air density increases, reducing the fin's effectiveness
   • B.TAS is higher, so the same angular disturbance produces a smaller proportional AoA change✓
   • C.The yaw rate gyro becomes less sensitive at altitude
   • D.Swept wing dihedral effect increases with altitude

   Why: Aerodynamic damping depends on the change in AoA produced per unit of angular disturbance. At altitude, TAS is much higher for the same IAS — a given angular displacement of the fin produces a proportionally smaller AoA change → smaller restoring aerodynamic force → less natural damping. See Section 2 . — This explains why yaw dampers are essential for jet transport operations but not needed for most piston trainers — the altitude and TAS differences are the critical factors.

6. Q6. During a yaw damper pre-engagement test, the position indicator moves to the left and then returns to centre. This indicates:

   • A.System fault — the indicator should not move during test
   • B.System serviceable — correct test response✓
   • C.LVDT failure — the indicator should stay at the deflected position
   • D.Phase advance circuit failure

   Why: During the yaw damper test, the test switch torques the rate gyro to simulate a yaw input. The system responds by deflecting the rudder (indicator moves in the tested direction). When the test switch is released, the LVDT feedback brings the rudder back to neutral (indicator returns to centre). Movement followed by return to centre = system functioning correctly. See Section 11 . — The two key checks in the yaw damper test: (1) indicator moves = gyro torquing → servo → rudder chain works; (2) indicator returns to centre = LVDT feedback → null position circuit works.