As in painfully slow, maddeningly slow, dangerously slow, for-freaking-ever slow. This generated a discussion of what the pilot in question is doing and why he is doing it.
The company I work for has adopted a rule that pilots on approach to land in a helicopter at any place other than a clear runway at an airport must not exceed a 200 foot-per-minute rate-of-descent during the last 300 feet of altitude. So, if you do the math, you can see that it should take 90 seconds to drop through the last three hundred feet.
That's a long time...
There are a several factors in this decision, and they are worth understanding. They include vortex-ring-state (of the main rotor system) also referred to as settling-with-power or power-settling; wire-strikes - our ability to see wires and avoid hitting them, or stop the aircraft when a strike is imminent; and being able to safely terminate the approach in a heavily loaded aircraft with marginal power reserves.
A helicopters rotor system operates in one of four modes, the normal thrusting state, when the engine(s) are driving the rotor and air is forced down through the rotor system, the autorotational state when air passes up through the rotor system and drives the rotor (as would happen after loss of power and rapid descent), the rotor-brake state which occurs in a tight turn or in a helicopter with an out of rig rotor system (rotor overspeeding), and finally the vortex-ring state or VRS.
In VRS, the rotor system is enveloped (from outside the disk around it's circumference and from the center) in swirls of air that destroy lift capability and result in a rapid sink rate. The houses get big fast. A pilot's normal instinct when entering into VRS is to pull more power to arrest the descent, but this aggravates the situation and makes the aircraft depart controlled flight, do crazy stuff, and fall faster. RW Prouty can give a more disciplined explanation of this of course, but hopefully you get the idea. I can explain this to most crewmembers by relating to the prop on a boat that is ventilating and loses all thrust. You have to slow down or stop and let water get back around the prop, and more power makes it worse.
|Image courtesy helidreams.com|
VRS is bad. The conditions that lead to VRS are a zero or near-zero speed through the air (not important how fast you are moving across the ground), some power applied (to get the vortex rings or "swirls" going - normally 20 to 100%) and a rate of descent 300 feet per minute or greater.
|The end result of Vortex Ring State...|
Now for the first 2.5 decades of my flying career, we made our approaches at the apparent rate of closure of a brisk-walk (using peripheral vision), and we would normally be around 45 knots airspeed descending at about 500 feet per minute when at 300 feet above the ground, gradually reducing speed and sink rate until touchdown. If you have been a crewmember for awhile, this is what you are used to. The brisk-walk rate-of-closure is still mentioned in the FAA's book on helicopters.
Generally, as long as one is not landing downwind, and has airspeed across the rotor system, VRS isn't a problem. The problem is that sometimes it's hard to tell where the wind is from, like at night on a middle-of-nowhere landing zone. Landing downwind puts the rotor system at zero airspeed before zero ground speed while still at altitude and permits the suction of air above the rotor system and the creation of the aforementioned vortex rings. But if we are descending at 200 feet per minute landing downwind doesn't matter so much.
That's one reason for the rule.
Then there is the fact that the slower you descend the more time you have to see a wire and avoid it. Pilots landing at scenes will instinctively land very slowly at night, and be looking for wires. During the day we can become complacent or distracted and not see a wire in our path until the last second.
If we see it.
By descending so slowly we afford ourselves the opportunity to STOP STOP STOP - and this has happened with a wire underneath the rotor disk in front of the windshield.
Finally, when we are heavy and the air is hot, our machines have little power reserve beyond what is required to fly. If we come in fast, there may not be sufficient power to arrest our descent and we can smack the ground. (Been there done that in a Chinook...) By approaching at 200 feet per minute we reduce the amount of extra power that will be required to terminate at a hover.
Now, sometimes the slow descent feels right (at night, dark scene, under goggles - or at max gross weight on a hot day - or in gusty winds), and then other times it feels unnecessary. And crews get impatient.
When we are nearly hovering at altitude, as is happening in a 200 fpm approach, we are inside the "dead man's curve" or avoid-area of the height velocity chart in the operator's manual. If the motor quits at that point, we are going to damage the aircraft and perhaps injure ourselves. There has been consideration of this risk, and it has been balanced against the other risks just mentioned, and flying in the avoid zone has been determined to be the lesser risk. They sign the front of the check.
The company has elected to make the slow approach the standard,in all cases except when landing at an airport, perhaps for positive habit transfer. I had a check airman have me demonstrate that I could do it recently, with the clock ticking off 90 seconds, and then he told me, "okay, the VSI (vertical speed indicator - indicates sink rate) on this ship is a little off so you can make your approaches just a bit faster." And so I did. Most of us know when to slow things down, but not everyone - so the rule is there for all of us...
|Photo courtesy Gibson and Barnes click here to visit...|