Other Dangerous Situations in a Helicopter

The flight dynamics, in a helicopter, may bring to some dangerous situations. The helicopter pilot has to now those at the effect of knowing which immediate and appropriate actions are to be performed to counter them, and avoid the loss of control of the aircraft

• the backwards blade stall. That situation occurs when the rotor's blades, when flying left of the rotor's disk -as seen from above the aircraft- reach their largest authorized angle of incidence, with respect to their profile. Which makes that they stall as they do not produce any lift on that side of the rotor. It's about the equivalent of a stall in a GA plane, for example. The backwards blade stall may occur in varied situations like when, first, the helicopter is flying at a high speed as the angle of incidence of the blades increases when flying backwards. Such that concept has yielded the notion of the maximal speed allowed for a helicopter. The backwards blade stall may occur too due to varied conditions. Turbulences. A feeble speed of the rotor. A abrupt and accentutated turn. Abrupt maneuvers. A important mass. Or a strong densimetric altitude (when flying from, around or to a airfield located in altitude on a warm day). The advisory hints that a backwards stall is closing are, among others, a low frequency vibration, a trend of the helicopter to nose pitch and a left turn. How to counter the situation? First, you will have to act immediately to avoid that the situation worsen! Just push the collective forwards so to decrease the angle of incidence of the rotor and to slow the aircraft
• the vortex effect. The vortex effect is the tendency that the helicopter has to swiftly descent inside the descending air column generated by the rotor downwards (which is also called the 'deflection' of the rotor). That is one of the most frequent cause of casualties aboard a helicopter. The descent rate inside the rotor's vortex is very high. To get into a vortex effect situation, the helicopter has to have a speed inferior to 10 kts, that he descends at a rate of more than 300 ft/mn and that he has a power superior to 20 percent. Such conditions generally occur either when in a stationary flight outside the ground effect and with no altitude maintained, either in case of trying to a stationary flight above the ground effect limit of the aircraft, or in the case of a rear tailwind landing (with the deflection vortex then pushed into under the helicopter). In such conditions the rotor does not produce anymore a lift sufficient to perform a stationary flight or a progressive descent. A vortex effect, in the flight deck is announced by a low frequency vibration, a loss of efficiency of the cyclical, and a strong descent rate. How to counter the situation? First, you will have to act immediately to avoid that the situation worsen! The helicopter must be taken out of the vortex with a motion forwards, backwards, or sidewards and at a speed of at less 10 kts!
• the G minus situation. The G minus situation occurs when the main rotor is raising less than 1G, which is less than the mass of the helicopter! The helicopter may find itself in such a situation in different cases. When the pilot abruptly uses the cyclical. When a helicopter is flewn into turbulences. When it is flewn on a parabolic trajectory (also said 'inverted trajectory') after a vertical climb. In these conditions of a G minus situation, the aircraft's nose is to stall, the helicopter may rapidly bank to the right due to the thrust of the tail rotor which, in such a situation, is now located above the main rotor, relatively as the main rotor further comes to hit the rear strut. That, generally, brings to the loss of both blades! How to counter the situation? First, you will have to act immediately to avoid that the situation worsen! Just pull gently the cyclical so to raise the nose back, thus adding Gs to the rotor's plane and, at the same time, have some input leftwards on the cyclical, which is counteracting the bank to the right!
• the one-sided blade stall for cause of a too slow rotation, the main engine failure, or the tail rotor failure are the other dangerous situations encountered with a helicopter. Both the latters are addressed in our tutorial 'The Autorotation Like the Engine Failure Procedure in a Helicopter'. All three necessitate too some immediate and appropriate action

Trainings for the dangerous situations in a helicopter have to be done in safe conditions, at a altitude sufficent enough to allow for that training. For each of the three conditions as described above, the helicopter should be placed in the said situation in the least dangerous way possible! For the backwards blade stall, the training will be made possible through a feeble rotation of the rotor only. To train for a vortex effect, the helicopter will be placed in those conditions through a stationary flight outside the ground effect and with no altitude maintained. As far as the G minus situation is concerned it seems impossible to safely place the helicopter in the required conditions. Thus the pilot will only have to know and memorize the situation and the counter-measures