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Helicopter Oral Exam Questions: Dynamic Rollover & LTE
Dynamic rollover and loss of tail rotor effectiveness (LTE) are the two ground-and-low-speed hazards every helicopter examiner probes, because both are entered through ordinary operations — a slope landing, a downwind hover turn — and both punish the instinctive correction. As with vortex ring state, the structure of a passing answer is: conditions, mechanism, recovery, prevention.
References: the Helicopter Flying Handbook chapters on hazards, your rotorcraft flight manual's slope limits and crosswind guidance, and (for LTE) the AIM and FAA advisory material describing the relative-wind regimes. Use your own aircraft's numbers — slope limits and pedal authority differ by type.
Dynamic rollover: the pivot and the critical angle
Dynamic rollover requires three elements: a pivot point (a skid or wheel in contact with the ground), a rolling moment about that pivot, and thrust (collective) applied. Once the aircraft starts rolling about a skid instead of its CG, lateral cyclic loses the ability to stop it — beyond the critical rollover angle (commonly taught as roughly 5–8°, varying with conditions and type), rollover is inevitable regardless of pilot input. Classic setups: a stuck or snagged skid at liftoff, slope operations beyond limits, lateral drift at touchdown, and right-skid-down situations in helicopters where tail rotor thrust and translating tendency add to the roll.
The recovery is counter-instinctive and absolute: smoothly lower the collective — reduce the thrust that is driving the roll. Pulling more collective or fighting with full lateral cyclic accelerates the rollover. Prevention: always lift off vertically and in trim with slow, deliberate collective application; confirm skids are free (mud, ice, frozen ground, tie-downs); respect RFM slope limits and abort a slope landing the moment cyclic reaches its lateral stop; and keep ground personnel and dollies clear of skids.
LTE: the three relative-wind regimes
LTE is an uncommanded, rapid yaw that occurs at low airspeed (generally below ETL) when the tail rotor cannot supply the anti-torque the moment demands — critically, it is not a mechanical failure and (as taught) not a true stall of the tail rotor. For a counterclockwise main rotor (American convention), the classic relative-wind regimes: weathercock stability (winds from roughly 120–240° relative — the tail tries to swing the nose into wind, and the aircraft can yaw suddenly if unchecked); main rotor disc vortex interference (winds from about 285–315° push main-rotor vortices into the tail rotor, varying its effective angle of attack); and tail rotor vortex ring state (left crosswinds, roughly 210–330°, where the tail rotor operates in its own recirculating wake, making thrust erratic). Add the aggravators: high density altitude, high gross weight, low airspeed, and high power demand — all of which raise anti-torque requirements and shrink pedal margin.
Recovery from an uncommanded right yaw (counterclockwise systems): full left pedal immediately, simultaneous forward cyclic to gain airspeed (flying through ETL restores effectiveness), and — if altitude permits — reduce power (lowering collective reduces torque and thus the anti-torque demand). If rotation cannot be stopped and a landing is inevitable, entering autorotation (throttle closed) removes the torque driving the spin. Prevention: know the wind at all times during hover work, avoid downwind hover turns and OGE pedal turns in the critical wind regimes, and maintain pedal margin awareness at high DA/weight.
Practice questions with answers & rationales
Q1. What three elements must be present for dynamic rollover?
Answer: A pivot point on the ground (skid or wheel in contact), a rolling moment about that pivot (drift, slope, stuck skid, crosswind, thrust asymmetry), and thrust applied via collective. Remove any one and you cannot dynamically roll over. That's also the logic of the recovery — collective down removes the thrust element instantly.
Q2. During a slope landing, the upslope skid touches and you're feeding in lateral cyclic. When do you abort?
Answer: The moment lateral cyclic approaches its stop before the downslope skid is firmly down — that means the slope exceeds your control authority (or the RFM slope limit), and continuing invites a roll about the upslope skid. Abort by smoothly returning to a hover (collective up cautiously, cyclic to level) and find a flatter spot. Quoting your RFM's slope limit and the rule 'never run out of cyclic with thrust on the rotor' is the expected standard.
Q3. Why won't full opposite cyclic save you past the critical rollover angle?
Answer: Once rolling about a skid, the rotor's maximum lateral cyclic moment is finite and the roll is being driven by thrust acting about a ground pivot — beyond the critical angle (~5–8°), the rolling moment exceeds anything cyclic can generate, and momentum carries the aircraft over. The only lever powerful enough is removing the driving force: lower collective. This 'why' is what separates a memorized answer from understanding.
Q4. What exactly is LTE, and is the tail rotor stalled?
Answer: LTE is an uncommanded rapid yaw at low airspeed that doesn't subside on its own, occurring when aerodynamic interference (weathercock winds, main-rotor vortex impingement, tail-rotor vortex ring conditions) plus high anti-torque demand exceed the tail rotor's available thrust or the pilot's timely pedal response. As taught in FAA materials, it is not a mechanical failure and not a conventional stall of the tail rotor — which matters, because the recovery assumes the tail rotor still works once conditions improve.
Q5. Which wind directions should make you most suspicious of LTE in a US-pattern helicopter, and why?
Answer: Left crosswinds around 210–330° relative (tail rotor vortex ring state — thrust gets erratic), winds from 285–315° (main-rotor disc vortices blow into the tail rotor, varying its AOA), and tailwinds from 120–240° (weathercock stability — the airframe wants to yaw nose-into-wind, and a right yaw can accelerate). Practically: during OGE hover and slow downwind turns, keep the wind's relative bearing in your scan and lead with pedal early — small, immediate corrections prevent the rapid-rate scenario.
Q6. You get a rapid uncommanded right yaw in a 50-ft OGE hover. Recovery?
Answer: Full left pedal immediately and forward cyclic to fly into translational lift — airspeed is the cure; and if altitude allows, reduce collective to cut the torque demand (mind the descent close to the ground). If the spin can't be stopped and ground contact is coming, autorotation (closing the throttle) removes the engine torque feeding the rotation. Afterward: the honest discussion of why you were hovering OGE downwind at high power in the first place — examiners want the prevention lesson voiced.
Q7. How do high density altitude and gross weight contribute to both hazards?
Answer: Both raise the power — and therefore torque — needed to hover, which raises the anti-torque (pedal) demand and shrinks the tail rotor's remaining margin (LTE side), and both put you closer to maximum collective where any rolling moment is amplified and margins for slope/stuck-skid mistakes are thinner (rollover side). At a high-DA pinnacle at max gross, you may have little pedal and little power margin left — performance planning and wind discipline are the actual mitigations.
Common mistakes to avoid
- Recovering from a developing rollover with more collective or just opposite cyclic — collective down is the recovery.
- Lifting off with a casual, brisk collective pull instead of a slow vertical liftoff in trim — that habit is the setup for stuck-skid rollover.
- Calling LTE a tail rotor failure or stall, then giving a mechanical-failure recovery. Different problem, different fix.
- Not knowing the three relative-wind regimes (or which side the critical crosswind is on for your rotor system).
- Ignoring pedal-margin awareness at high DA/weight — examiners build their LTE scenario at a hot, high, heavy pinnacle on purpose.
- Doing downwind hover turns in the scenario without comment. Mention wind every time you describe hover work.