In the very early days of my aviation education there were five hazards of helicopter flight that were drilled into me: Dynamic rollover, ground resonance, retreating blade stall, low G – Mast bumping and settling with power. Every check ride oral evaluation from 1989 to the present day has included questions about most if not all of these hazards. Questions about how to recognize them, how to avoid them and what to do if you inadvertently encounter them. It was a solid education early on that has served me well over the last 33 years and particularly over the last 20 years flying in the helicopter air ambulance (HAA) industry. I do wish I could say that I never once allowed Murphy1 to enter the cockpit, but once upon a time on a dark moonless night, I did.
Over the last 20 years I’ve flown over 5000 patients - about half of those flights were at night and half of those were to unimproved areas (streets, highways, fields etc). In that time, I’m humbled to say that I had not had a single accident; however, one dark night on a scene call to a narrow cul-de-sac, feeling pretty confident about myself as a pilot, I nearly ignored Murphy’s Law1 in favor of my own arrogance, and set a course of singular events into action that when combined placed my aircraft, my crew and myself in a completely avoidable situation. One that in the absence of a little dumb luck and divine intervention, could have had a far worse outcome.
All in the approach
When setting up for any approach, particularly one to an unimproved area, a high reconnaissance is flown at a safe altitude but generally no less than 500 feet above ground level. Once identifying the landing area, you fly an orbit to identify any hazards, obstructions or landing restrictions. A sterile cockpit is observed, where no non-essential communication takes place. Any safety concerns and crew members are expected to speak up immediately. Crew coordination is essential and crucial during this operation, and every crew member has a say as to the suitability of the area for landing. All crew members have eyes outside and call out anything that presents a safety concern.
Absent any concerns, the helicopter sets up on downwind and reduces airspeed to an amount suitable for the approach. As the aircraft turns its base leg and then onto final, the descent is begun and a low reconnaissance is performed confirming everything observed during the high recon. At that time I continue to slow the helicopter and set an approach angle that allows me to keep the landing area in sight, clear all obstructions in the approach path and before descending below the highest obstacle in the departure path of the landing zone and its surrounding area, make a final decision to continue or not.
A common misconception regarding helicopter flight is that they can just descend vertically into a landing area with little or no forward airspeed. On an intellectual level that makes sense, but from an aerodynamic level it’s not as easy as that. In very basic terms helicopters obtain their lift by the rotation of their rotor blades, which have very specifically engineered shapes. Much like the wing of an airplane as it is accelerated forward, the rotation of the rotor blade creates a lower pressure area above due to the accelerated airflow over the curved surface of the blade and a higher pressure below the blade. In a ‘no wind’ hover, this pressure differential creates a circulating pattern, whereas the higher pressure below seeks to fill the lower pressure above and a circulating pattern develops. If in this condition the aircraft is allowed to descend at a rate of 300 FPM or greater, it is essentially settling down into its own downwash – and that rate of descent can increase rapidly.
In a wind above 16 knots or in forward flight (or a combination of wind and forward flight) fresh airflow goes onto the rotor system dispersing these recirculating patterns of air, the rotor becomes more efficient and is able to fly away. In descending vertically or nearly vertically, as you descend into the column of recirculating airflow, and the higher pressure generated beneath the rotor circulates around and above the rotor, and can actually result in a downward force to the aircraft. The natural reflex upon recognizing this condition would be to pull in more power and raise the collective – but by this time, that only exacerbates the problem. Essentially you’ve further increased the circulating airflow and increased the downward force on the helicopter. To get out of this condition you must do something very counter intuitive – lower power, reduce collective (altitude permitting) to reduce the downward force, and increase airspeed to fly into clear, undisturbed air.
Helicopters don't fly, they beat the air into submission
The Devil on your shoulder
On this night I allowed myself to turn in tight to the landing zone at a higher rate of speed than I should have, and instead of responsibly making a ‘Go Around’ decision, climbing and flying upwind, extending the downwind and allowing myself to get myself better set up for a nice, controlled approach, I let my ego have some input. I attempted to correct for my sloppy high reconnaissance with a steeper rate of descent and slower approach speed. The high ROD quickly accelerated, and the inevitable aircraft shudder caught me by surprise. For a moment the sight of the fire truck, ambulance, and police vehicle created an urgency in me to complete the approach. It was not unlike having the often-used comedy cliché devil figure on my shoulder, whispering taunts in my ear: “You can make it. Keep going. It will be fine. You’ve got this.” I almost let me ego chide me into continuing.
An instant later, on the opposing shoulder, an angel symbolically appeared and countered with “Go around dummy; pull power, climb, do over. This won’t end well”. As we were rapidly descending below that highest obstacle in the departure path, the time to decide was now. I grabbed as much collective as I could and accelerated to Vy (Best Rate of Climb). Over the radio, I could hear the emergency medical service (EMS) team saying ‘where are you going?’ with a bit of confusion mixed with some derision, or so it felt. I keyed up and simply stated, “Pilot error, I’ll be coming around again.”
Avoiding settling with power (SWP) is as simple as avoiding the conditions required for it (a 300 feet per minute rate of
Avoiding SWP is as simple as avoiding the conditions required for it
descent or greater, with little to no forward airspeed (less than effective translatable lift), using 20 to 100 per cent of available engine power). A good rule of thumb is to never let your rate of descent approach 300 feet per minute when within 300 feet of the ground. Always set up your approach with a headwind as much as conditions allow. Be aware of the early onset warning signs (aircraft shuddering). Monitor your Vertical Speed Indicator. Plan your approach early and allow for time to get the aircraft configured for a smooth transition on approach and at the first sign of entering SWP, reduce power (altitude permitting), accelerate, and fly out of it - forward cyclic and as soon as you’re clear of vortex ring state, re-apply power and climb.