A COMPARISON OF
HEMS TO OTHER
TYPES OF AVIATION
Having reviewed HEMS-specific data, this report will now compare HEMS accident and incident data to other types of aviation. This section first looks at two reports and then focuses on various aviation industry statistics. Finally, the accident rates for helicopter air medical transport is compared to other aviation operations.
HELICOPTER EMS vs. ALL
HELICOPTER ACCIDENT
DATA: 1990–2000
During the 2000 Air Medical
Transport Conference (AMTC), Sandra Hart of the NASA-Ames Research Center presented a study that compared characteristics of helicopter EMS acci- dents with those of all helicopter acci- dents (EMS and non-EMS) over the Figure 1–32: HEMS Accident Rate per 100,000 Patient Transports, 1972–2000
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 Year A c c ide n t R a te pe r 1 00 ,0 00 Tr a n s por ts
this same time frame (1982–1987), our statistics yield a much higher accident rate of 15.58 per 100,000 flight hours and a fatal accident rate of 5.2.
Percentage of HEMS
Programs and Helicopters
Involved in Accidents
Data is not available to accurately determine what percentage of HEMS pro- grams have sustained an accident. Over nearly 30 years of civilian HEMS opera- tions, dozens of programs have closed and others have merged operations. However, with the data that we have accumulated to estimate the number of programs and
helicopters each year, we can determine annual percentages with some accuracy. Averaging these annual calculations, we can estimate the overall percentage of HEMS programs and helicopters that have had accidents. These calculations do not take into account the possibility that an individual program may have suf- fered more than one accident—which has occurred.
The calculated percentage of programs and helicopters that sustained accidents each year since 1980 is shown in Figure 1–29, Lines 18a and 18b. The five acci- dents that occurred prior to 1980 are not included, as we do not have annual statis- tics on the number of programs or heli-
0% 2% 4% 6% 8% 10% 12% 14% 16% 18% 20% Year Pe rc e n ta ge Programs Helicopters
same time period. In March 2001, Hart presented an updated report at the Proceeding of the 11th International Symposium on Aviation Psychology.
In her introduction at AMTC, Hart pointed out that in recent years, the number of HEMS accidents has
increased. However, she emphasized that all we have is raw numbers. Accurate exposure data is lacking regarding the number of transports and the number of hours flown by helicopter EMS. Without this, we do not have accurate informa- tion to determine if the increase in acci- dents is related to an increase in the hours flown or whether HEMS had got-
ten less safe. As a result, the data pre- sented in her lecture was based upon raw numbers and percentages rather than as accident rates.
In addition, Hart stated that “aircraft accidents are poor indicators of safety trends due to a low occurrence rate and limited information about what hap- pened. Quite often, the immediate ‘cause’ may have little relationship to the underlying causes. But as risk factors begin to accumulate, something bad was bound to happen.”
An analysis of 1,494 helicopter acci- dents over a 10-year period beginning in 1990 was conducted by the NASA-Ames
Research Center. The database was obtained by reviewing the NTSB acci- dent and incident reports, looking at nar- ratives, probable cause, occurrences, find- ings and coded data (e.g., pilot experi- ence, helicopter make/model, visibility, mission). A total of 58 HEMS accidents were identified within this database— both Part 135 (patient-related transports) and Part 91 flights (repositioning, main- tenance, etc.). Of interest, 72% of the HEMS accidents were flown under Part 91 and only 30% were on patient-related missions. Some HEMS operations con- duct the flight to the patient under Part 91 regulations, while other programs consider all legs of a patient flight to be Part 135.
Figure 2–1 illustrates the number of accidents that were included for each year in the Ames report. Several 1999 and 2000 accidents lacked NTSB final reports and were not included.
When the Accidents Occur
When comparing the data, approxi- mately 53% of the HEMS accidents occurred between dusk and dawn while only 9% of all helicopter accidents occurred at night. It is estimated that only 5% of all helicopter flights are at night.
Hart found that nearly five times as many HEMS accidents occurred in IMC conditions (24% compared to 5% for all helicopter accidents), with nearly all of them involving inadvertent flight into IMC conditions. This was consistent with the previous NTSB study. Neither snow nor rain was an identified problem, with very few helicopter accidents occur- ring with visible precipitation. This could be due to the fact that fewer heli- copter flights occur during these types of weather conditions. Another possibility could be that pilots are more careful when they can see visible precipitation (rain or snow). In contrast, deteriorating weather conditions (decreasing ceilings or fog developing) may be less obvious.
Of the 58 accidents, weather condi- tions were cited as a contributing factor 29 times. It is important to note that these were factors and not the cause of the accidents. In some cases there was more than one weather factor cited with 0 1 2 3 4 5 6 7 8 9 Year A cci d e n ts
FAR Light Condition Weather Precipitation
72% 28% 0 53% 47% 24% 76% 12% 82% 68% 9% 21% 11% 89% 5% 95% 19% 81% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Part 91 Part 135 Other Dusk to Dawn
Day IMC VMC Snow,
Rain, Sleet
None
HEMS All Helicopter
Figure 2–1: Number of HEMS Accidents per Year
Adapted from: Hart, Conference presentations, 2000/2001
Figure 2–2: When Accidents Occur (HEMS: n=58; All Helicopters: n=1,494)
regard to an accident or incident. Weather is therefore likely to add to the risk of a flight. It may not cause the acci- dent, but it may increase the likelihood that an accident could occur. Weather was cited only twice as the cause of an accident.
With regard to the phase of flight, there are some significant differences when comparing HEMS to all helicopter accidents. For all helicopter accidents the highest percentage of accidents were seen during landing (25%), maneuvering (21%), cruise (15%), hovering (11%), and takeoff (11%). This is significantly different from HEMS accidents that occurred most commonly during cruise (24%), takeoff (19%), approach (16%), and landing (14%). Figure 2–3 shows the distribution of accidents during the various phases of flight.
Pilot Experience
Experience of the pilots was carefully reviewed by the Ames study. In general the HEMS pilots averaged slightly fewer total hours than the total pilot database (6,307 vs. 6,424). However 79% of the HEMS pilots’ hours (5,010) were in heli- copters. The overall group included commercial fixed-wing pilots who did much less time in helicopter aviation (66% or 4,230 hours). HEMS pilots averaged fewer hours (753) in the make/model helicopter they were flying at the time of the accident compared to pilots for all of the helicopter accidents (1,273). In addition, unlike the NTSB report, pilot fatigue was not found to be a significant factor in these HEMS acci- dents. According to the study, the aver- age HEMS pilot had flown only 1.88 hours in the 24 hours prior to their acci- dent which was less than the average (3.00) for the “all helicopter accident” group.
The Ames researchers found a signifi- cant difference with regard to instrument ratings. EMS pilots were far more likely to have an instrument rating than all hel- icopter pilots involved in accidents. They may not have been current and they may not have been flying helicop- ters that were IFR equipped, but their training and experience was noted. According to Hart, while this additional training and experience should be con-
sidered an advantage to the EMS heli- copter pilots, it may have worked as a disadvantage if the pilots felt that their training and experience would allow them to “push the envelope a little bit more.”
Vehicle Characteristics
There were more HEMS accidents involving twin-engine helicopters than single-engine helicopters (63% vs. 36%, respectively). This is in contrast to all the accidents where the majority of the aircraft were single-engine aircraft (89%). It is important to note that no mention or comparison is made with regard to the percentage of single- vs. twin-engine hel- icopters in operation—only the compari- son of those involved in accidents.
Figure 2–3: Phase of Flight (HEMS: n=58; All Helicopters: n=1,494
Adapted from: Hart, Conference presentations, 2000/2001
0% 0% 19% 3% 24% 7% 16% 14% 9% 7% 3% 1% 11% 1% 15% 5% 6% 25% 21% 11% 0% 5% 10% 15% 20% 25% 30% Sta nding Tax i Tak e-of f Clim b Cru ise Des cent App oach Land ing Man uver Hover
HEMS All Helicopter
EMS All Helicopter
Average Range Average Range
Total Hours 6,307 3,000–19,275 6,424 29–34,886 Helicopter Hours 5,010 27–17,380 4,230 8–25,000 Hours in Make 753 16–3,620 1,273 3–8,918 Instrument Hours 269 0–1,647 203 0–3,613 Prior 24 hours 1.47 0–6 3.00 0–15
Accident Characteristics
HEMS accidents have a much higher likelihood of resulting in serious injuries or fatalities than other helicopter acci- dents. Of the 58 accidents studied, 38% resulted in at least one fatality compared to 17% for the overall database. These results are similar to the previous studies. While there tended to be very few post- crash fires in either group, aircraft in both groups were either destroyed or seri- ously damaged at very high rates (92% for HEMS vs. 97% for all accidents). HEMS aircraft, however, were destroyed at a much higher rate. Figure 2–5 shows the accident characteristics for both groups.
Figure 2–4: Pilot Experience
Figure 2–5: Accident Characteristics (HEMS: n=58; All Helicopters: n=1,494)
Adapted from: Hart, Conference presentations, 2000/2001
5% 2% 88% 35% 14% 10% 42% 45% 47% 5% 3% 1% 10% 89% 18% 14% 22% 64% 26% 71% 1% 2% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Fire In-fligh t Fire on Grou nd No Fire Fatal Ser iou s Mino r Non e Des troye d Ser iou s Mino r Non e
HEMS All Helicopter
Pre/Post Crash Fire Severity of Injuries Aircraft Damage
9% 2% 5% 28% 16% 14% 12% 10% 2% 6% 13% 1% 6% 12% 8% 3% 17% 27% 3% 10% 0% 5 % 1 0% 1 5% 2 0% 2 5% 3 0% Airframe/System Failure Fire / Explosion Hard Landing Inflight Collision w/Object Inflight Collision w/Terrain Inflight Encounter
w/Weather Loss of Control Inflight
Low Engine Power Roll Over Misc. / Unknown
All Helicopter HEMS
Figure 2–6: First Events (HEMS: n=58; All Helicopters: n=1,494)
Adapted from: Hart, Conference presentations, 2000/2001
Chain of Events
and First Events
Hart’s report emphasized that accidents are not caused by a single event. In most accidents, numerous risk factors can be identified. An accident might have an obvious identifiable cause, but there are likely to have been numerous risk factors that contributed to the event or the severity of the event. Acting on any of these risks might prevent an accident from happening or lessen the severity of the accident.
When the NTSB looks at accident data, they pay particular interest to the “first events”—the first obvious and measurable event that can be considered an accident. It is not the first occurrence in the chain of events that leads to the accident. Rather, it corresponds to the first event that would be considered evi- dence that an accident has taken place. First events are not causes. Their analysis tells us what has happened, but do not tell you why it happened.
The most common first event in HEMS was an in-flight collision with ter- rain, with wires being the primary offend- er. EMS helicopters were more than twice as likely to strike an object or ter- rain and nearly five times more likely to have an encounter with inclement weather. Relatively few of the HEMS accidents were caused by low engine power, airframe, or component failures compared to the all helicopter accident group. Hart concluded that this seems to suggest that, in general, EMS helicopters are well maintained.
The Ames study presented a cascading chain of events of what happens when an accident occurs. An in-flight encounter with weather is rarely the cause of an accident but is often the first event. It leads to an in-flight collision with an object or terrain or to a loss of engine power. Another frequent first or second event is loss of control in flight, which may follow some type of system failure (which is rare).
Cause and Contributing
Factors of Accidents
When the NTSB finalizes their acci- dent reports they try to identify a cause of each accident. This is not necessarily the only event that contributed to the acci-
dent. Pilot-related factors (human fac- tors) were cited 50 times in the 58 acci- dents. These factors included operating with known deficiencies, inadequate pre-flight planning, inability to evaluate the weather, inadvertent flight into IMC, failure to follow procedures, spatial disorientation, lack of experience, and
failure to maintain proper speed, alti- tude, rate of descent or climb, or RPM. Of interest, Hart pointed out that in very few of these accidents was it identi- fied that the pilot deviated from the FARs or company regulations. This is significantly different than the data reviewed earlier in the ASRS report.
Aircraft-related causes of the 58 HEMS accidents were cited 22 times, while fuel-related problems were cited twice. Weather was cited only once.
There are a number of contributing factors that increase the risk for an acci- dent. Many of these are related to weather (e.g., icing conditions, clouds, fog, rain, snow, sleet), which was cited 29 times as a contributing factor in the 58 accidents. Person-related contributing factors were cited 25 times, which included distractions, pressure felt by the pilot, as well as the items listed under “causes.” Terrain, which was cited 24 times, was another consideration and fly- ing at night (dusk to dawn) was cited 18 times.
FACTORS RELATED TO
OCCUPANT CRASH
SURVIVAL IN EMS
HELICOPTERS
Much of what we have been reviewing has dealt with the associated risks and events surrounding accidents that have occurred in the past. It is also essential to assess the relative occupational risk to the crewmembers of EMS helicopters. Robert Dodd’s 1992 Ph.D. dissertation evaluated the incidence and seriousness of crash-related injuries among EMS hel- icopter occupants in survivable crashes.
The study found that main cabin occu- pants in EMS helicopters have nearly 4.5 times the risk of serious injury or death in survivable crashes when compared to a comparable population of occupants in the main cabin of non-EMS air taxi heli- copters. For front seat occupants, he found that there was no significant differ- ence in injury risk between the two groups. This seemed to support his prem- ise that EMS aircraft modifications, which are generally limited to the main cabin, were directly associated with the risk of injury and may contribute to occu- pant injury and death in otherwise sur- vival crashes.
For his study, Dodd reviewed 75 EMS accidents from 1978–1983 and
1983–1989 (with 241 occupants) and 147 non-EMS helicopter accidents from 1983–1989 with 485 occupants. His comprehensive review and analysis of
available reports identified survivable vs. non-survivable crashes and occupants, as seen in Figure 2–7.
Comparing occupant location in sur- vivable accidents with specific injury pat- terns, Dodd concluded that EMS main cabin occupants were at a higher risk for serious back injuries and serious head injuries. Figure 2–8 compares the per- centage of occupants with specific injuries and where they were seated in the helicopter.
Dodd’s research included written sur- veys that were sent to survivors. Twelve injured EMS occupants indicated injuries that were the result of striking medical equipment inside the helicopter during the crash. This equipment included the stretcher, cardiac monitor, medical panel, oxygen tanks, and portable radio.
As part of his research, Dodd evaluated numerous variables to determine how they had influenced injuries. The vari- ables included crash severity, post-crash fire, number of engines, helicopter weight, light conditions, use of shoulder harness, cause of the crash, age, and sex. For each of these variables, he calculated the relative risk of injury for EMS occu- pants compared to non-EMS occupants in survivable crashes. Dodd found that there was a significantly greater risk of injury (significant relative risk) in HEMS accidents compared to non-EMS acci-
dents: where there was a post-crash fire; in single-engine helicopters; with heli- copters weighing < 4,500 pounds; during daylight conditions; with EMS occupants who did not wear a shoulder harness; and mechanical-related crashes. There was a marginal relative risk of injury in heli- copters that weighed > 4,500 pounds.
In contrast, he found no significant rel- ative risk of injury in: the absence of post-crash fire; twin-engine helicopters; during dark (night) conditions; or crashes caused by loss of control, wire strikes, or bad weather. Even though significantly more EMS accidents (32%) occurred in bad weather compared to the non-EMS study group (14%), there was not a sig- nificant increase in the risk of injury. There was also no correlation between injury and the age or sex of the occu- pants between the two groups.
Dodd also evaluated the severity of the accidents and classified them as Crash Severity Level 1 (hard landing), Crash Severity Level 2 (hard landing with sub- stantial damage), and Crash Severity Level 3 (high vertical impact or cruise collision with ground). Level 1 accidents resulted in no injuries to EMS occupants and only one non-EMS injury. Level 2 accidents yielded a significant relative risk for passengers in the main EMS cabin, but a non-significant risk for front-seat passen- gers. In Level 3 accidents, the significant
Serious Back Serious Head Minor Head Internal Fractures Injuries (%) Injuries (%) Injuries (%) Injuries (%) (%)
Occupant EMS Non- EMS Non- EMS Non- EMS Non- EMS Non-
Location EMS EMS EMS EMS EMS
Pilot 4.5 10.8 2.3 2.4 0 3.6 0 0 2.3 7.2
Front Seat 15.4 2.7 7.8 5.4 0 2.7 11.5 0 3.8 8.1
Main Cabin 28.6 9.2 14.3 1.3 5.3 1.3 7.1 2.6 10.7 5.3
Patient 14 N/A 7.1 N/A 0 N/A 0 N/A 7.1 N/A
Total Non-survivable Survivable
Crashes Occupants Crashes Occupants Crashes Occupants
EMS 75 241 24 70 51 171
Non-EMS 147 485 20 50 127 435
Figure 2–7: Survivable and Non-survivable EMS and Non-EMS Crashes
Adapted from: Dodd, unpublished dissertation, 1992
Figure 2–8: Distribution of Injuries in Survivable Crashes—EMS vs. Non-EMS Crashes
relative risk for main cabin occupants was even greater in EMS aircraft.
Dodd’s calculations of accident rates were similar to the NTSB report that had come out four years earlier. He found that EMS helicopters had an accident rate of 11.84 per 100,000 hours of flight. This is more than 2.5 times that of non- EMS air taxi helicopter operations that were found to have an accident rate of 4.43 per 100,000 flight hours. While Dodd did not compare fatal accident rates, he did compare the percentage of occupant fatalities for his two study groups. He found that 32% of the EMS occupants suffered fatal injuries, while only 9% of the non-EMS occupants died —a rate that is 3.5 times greater for the EMS group.
Dodd’s report makes a convincing statement for addressing accident survival as well as accident prevention. EMS per- sonnel are injured more frequently and more severely in survivable HEMS acci- dents when compared to occupants of non-EMS helicopter crashes.
Dodd presented a 1991 study by Crowley that evaluated the use of hel- mets in survivable military crashes. Crowley found that occupants without helmets were 4 times more likely to suffer a serious head injury and 6 times more likely to suffer a fatal head injury than occupants with helmets. Limiting the comparison to the main cabin increases the risk. Main cabin occupants with no helmet were found to be at 5 times the risk for a serious head injury and 7.5 times the risk for a fatal head injury than their helmeted counterparts. While Crowley’s study was based on military data, he suggested that it might also be applicable to survivable HEMS accidents.
Another series of studies for the U.S. Army concluded that back injuries were the most common injury suffered by