Analysis of an accident report.
The other day, I got a call from an off-road racing team manager. He was interested in hiring me to chase his truck at an upcoming race. But rather than ask me the usual bunch of questions about rates and ferry times, he grilled me about my flight experience.
How long have you been flying? Do you own the helicopter? How many times have you done this?
I answered all of his questions honestly — I have nothing to hide. I’ve been actively flying for about 8 years, since I bought my first helicopter. I have about 1800 hours of flight time. I own the helicopter I fly and I’ve put all of its 610 hours on it. I’ve followed cars and trucks and boats during races about ten times now.
Then he asked, “Did you hear about the helicopter crash at Lucerne Valley last year?”
I told him I hadn’t.
“One of our guys was on board. The pilot didn’t have very much experience, so we’re careful about who we hire now.”
The Accident Report
Later, after discussing rates and finishing up the call, I looked up the info on the accident he referred to in the NTSB database. I found it under NTSB Accident number LAX05FA189. He’d been wrong about the accident date — nearly three years had passed since the May 28, 2005 accident in Lucerne Valley, CA. The helicopter had been a Robinson R44. Although no one had died, three people had been seriously injured in the crash.
I read accident reports to find out how accidents occur. This helps me stay aware of potential problems with my aircraft or flight situations. Here’s the short text of this particular accident’s description:
The helicopter impacted level terrain and rolled onto its left side while maneuvering during a low-level photo flight. The pilot was flying southbound along a racecourse on a photo flight when he made a hard 180-degree turn, and lost control of the helicopter. As the helicopter began a spin to the right, the pilot noted a loss of rpm and altitude. He asked the certified flight instructor (CFI)/safety pilot to take the flight controls. As the CFI took control of the helicopter, he realized that the rpm’s were decaying and that the helicopter was too low to recover the rpm. He attempted to cushion the impact with the collective. Both pilots reported that there were no preimpact mechanical anomalies with the airframe or engine. No evidence of any preimpact mechanical malfunction was found during the post accident investigation that would have precluded normal operation. During the accident flight, the helicopter was running approximately 131 pounds over the maximum gross weight for an Out of Ground Effect hover for the existing atmospheric conditions. The helicopter was being operated in a high density altitude environment, which was computed to be 7,350 feet mean sea level. The pilot received his rotorcraft helicopter rating 7 days prior to the accident. He had approximately 77 hours of total rotorcraft flight time at the time of the accident. The helicopter manufacturer indicated that photo flights were a high risk phase of flight and issued a safety notice SN-34. Safety Notice SN-34 recommended at least 500 hours and extensive training in both low rpm and settling-with-power recovery techniques prior to flying photo shoot type flights.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
the pilot’s failure to maintain adequate main rotor rpm and directional control while maneuvering at low altitude. Contributing factors in the accident were the helicopter’s gross weight in excess of the maximum hover out of ground effect limit, a high density altitude, and the pilot’s lack of overall experience with regard to low rpm and settling-with-power recovery techniques.
There’s a lot of information here. I’ll review it and explain the parts that might not make sense to folks who aren’t pilots, aren’t helicopter pilots, or are new helicopter pilots.
Out of Ground Effect Hover
Let me start out by explaining that helicopters need more power to hover than to perform just about any other maneuver. That might seem counter intuitive, since when you’re hovering, you’re really not going anywhere. But the reason for this is that when a helicopter is in motion, there’s an increase in lift from the relative wind against the rotor blades.
From the FAA’s Rotorcraft Flying Handbook:
Translational lift is present with any horizontal flow of air across the rotor. This increased flow is most noticeable when the airspeed reaches approximately 16 to 24 knots. As the helicopter accelerates through this speed, the rotor moves out of its vortices and is in relatively undisturbed air. The airflow is also now more horizontal, which reduces induced flow and drag with a corresponding increase in angle of attack and lift. The additional lift available at this speed is referred to as “effective translational lift” (ETL).
So you use less power to fly than to hover. And any speed lower than ETL will require more power than speeds above ETL. Helicopter pilots often use 30 knots airspeed as a rule of thumb.
You can break down hovering into two types: in ground effect (IGE) and out of ground effect (OGE). From the Rotorcraft Flying Handbook:
When hovering near the ground, a phenomenon known as ground effect takes place. [Figure 3-7] This effect usually occurs less than one rotor diameter above the surface. As the induced airflow through the rotor disc is reduced by the surface friction, the lift vector increases. This allows a lower rotor blade angle for the same amount of lift, which reduces induced drag. Ground effect also restricts the generation of blade tip vortices due to the downward and outward airflow making a larger portion of the blade produce lift. When the helicopter gains altitude vertically, with no forward airspeed, induced airflow is no longer restricted, and the blade tip vortices increase with the decrease in outward airflow. As a result, drag increases which means a higher pitch angle, and more power is needed to move the air down through the rotor.
Here’s the diagram that goes with this information:
Operating Handbooks for helicopters provide charts that help pilots determine the expected performance of the aircraft in a variety of conditions. The accident report indirectly refers to the OGE Hover Ceiling chart. This chart tells a pilot the maximum altitude you can expect to maintain an out-of-ground effect hover given the aircraft weight, altitude, and outside air temperature. The one shown here is for a Robinson Raven I helicopter (I can’t seem to find my office copy of my Raven II manual). Here’s how it works. Start by following the weight line up to where it hits the temperature line. Then follow that intersection across to the altitude.
For example, if the aircraft was 2300 pounds and the outside temperature was 20°C, the maximum OGE hover would be 3,000 feet pressure altitude. In this accident, the aircraft was roughly 2,220 pounds at the time of the accident and the temperature was about 31°C. Following those lines on the chart indicates that the maximum OGE hover would be 3,100 feet pressure altitude.
The accident location was at 4,266 feet MSL. So the pilot was operating in an area and at a weight that made out of ground effect hover impossible in his aircraft.
Now I don’t want people reading this to think that you can’t (or shouldn’t) fly a helicopter in a place where you can’t hover out of ground effect. That’s not true. But the OGE hover situation does weigh heavily into this accident because of the maneuvers required for the mission.
Lower Rotor RPM
Having just flown with a photographer for an off-road race, the kinds of maneuvers needed are very fresh in my mind. More than once, we were required to slow down to wait for a vehicle. We also hovered OGE several times, with and without slight tailwinds or crosswinds. And, of course, we often had to make sudden course changes that required sharp 180° turns. These are not simple maneuvers, especially when power is an issue.
I can also say from experience that when flying an R44, if you pull more power than what is available, the first indication of a problem will be a low rotor RPM horn. I’ve had this happen twice on takeoffs at high altitude locations (over 6500 feet) with four people on board. If you can increase airspeed to reduce the amount of power needed to fly, you can get out of the situation, but that normally means a descent. (In one case, I did a running take-off from an airport and was fine once airborne; in the other case, my takeoff was from the edge of a cliff, so a descent wasn’t a problem.)
Here’s how I see this accident setting up. The pilot is going very slowly, below ETL, almost in a hover, and makes a hard turn. The aircraft starts to settle, so he pulls more pitch. This increases the drag on the main rotor blades, but there isn’t enough power to overcome it. The blades slow down. The low rotor RPM warning system sounds its horn. He’s too close to the ground to push the cyclic forward and get the airspeed he needs to get out of the bad situation.
How could this have been prevented? One way is to lighten the load. The maximum gross weight for a Robinson Raven I is 2,400 pounds. The pilot and passengers weighed 230, 180, and 175 respectively. If the pilot had checked the OGE charts, he would have seen that the aircraft was too heavy to fly at speeds less than 30 or so knots. He could have taken on less fuel or, better yet, flown without the 180-pound man beside him.
Another way to prevent the problem is to keep the aircraft speed up, above a minimum of 30 knots. This will prevent the pilot from getting into a situation where OGE hover power is required.
I should note here that I considered buying a friend of mine’s R44 Raven I — until I looked at the OGE hover chart. The performance was simply not acceptable to me. I often fly to the Grand Canyon (6300 feet), our vacation property (6700 feet), and Sedona (5200 feet) on hot days. Based on the chart, it was uncertain whether I’d be able to land and take off from these destinations when the aircraft was near maximum gross weight. I needed the additional power and performance of the Raven II for flexibility and safe operation in these areas.
Like my potential client, I think a main contributing factor to this accident was the pilot’s inexperience. He had only 70 hours in helicopters, and this was the first time he’d flown this kind of helicopter at a race event. He simply was not prepared for the kind of maneuvers he’d have to perform to get the job done. Add that to the OGE hover problem and it’s easy to see how this could have happened.
Robinson Helicopter Company knows that inexperienced pilots should not be flying photo missions. It issued Safety Notice SN-34 in March 1999 which states:
There is a misconception that photo flights can be flown safely by low time pilots. Not true. There have been numerous fatal accidents during photo flights, including several involving R22 helicopters.
Often, to please the photographer, an inexperienced pilot will slow the helicopter to less than 30 KIAS [knots indicated airspeed] and then attempt to maneuver for the best picture angle. While maneuvering, the pilot may lose track of airspeed and wind conditions. The helicopter can rapidly lose translational lift and begin to settle. An inexperienced pilot may raise the collective to stop the descent. This can reduce RPM thereby reducing power available and causing an even greater descent rate and further loss of RPM….
The Safety Notice goes on to recommend that the pilot have at least 500 hours pilot-in-command time in helicopters and over 100 hours in the model flown before conducting photo flights.
I’m glad my potential client mentioned this accident and I’m very glad I looked it up. I learned a lot from reading it, analyzing it, and summarizing my thoughts here.
It’s also made me more aware of weight and performance at events like these.
When you do a lot of point-to-point flying, you become somewhat complacent about the aircraft and don’t consider the additional demands of multi-maneuver flying. Although I’m always concerned with the weight of my aircraft on takeoff, I tend to look at it more in terms of whether I’m too heavy to fly legally — over maximum gross weight. With only 3 people on board, unless we’re all fatties, that’s not usually a concern.
Parker was at less than 500 MSL and it was a cool day, so I admit I didn’t check the hover charts. (When you check them over and over, you get a “feel” for them and can “guesstimate” what they’ll say.) But after reading this and thinking about it, I’ll review the charts before each photo flight, even if I’m already pretty sure that OGE operations won’t be a problem.
Any thoughts, comments, experiences you want to share? Use the Comments link or form for this post.