High Altitude Case Study
To further discuss preflight performance planning and initial on scene actions and considerations, I will use the following case study from a high, hot hoist from Mt. Washington in central Oregon:
Initial Notification: On a Saturday, around 1530L, the Oregon State SAR coordinator called with a request for helicopter support to hoist a climber on Mt. Washington (IVO Bend, OR) at 7500’. The climber had been injured by a falling boulder and there was concern of significant fractures and internal bleeding. The initial assessment was that timeliness was important.
Alternatives: In this case, there were no suitable alternatives. Terrain was rough and there were no options to land (e.g., helicopter EMS alternative). A ground rescue would require a carryout, necessitating another night on the mountain. In Oregon, National Guard H60s are the only other hoist capable response asset, but because they do not maintain a ready crew, the SAR coordinator felt lead time to gather a crew would be too long.
Location Recon: Although there is an abundance of different tools to gather information about location specifics amid an inland case, the two I rely on most is the GAIA app and Google Earth.
GAIA allows me to locate the mountain, confirm the elevation, and evaluate terrain. It also has detailed road, trail heads, trails, and shelter information, which is useful when coordinating efforts with other SAR entities, particularly ground parties.
Google Earth is helpful because it provides a preview of the potential hoist area, including any hazards that may be present (e.g., will the hoist take place in tall trees or a wide-open barren area). It also makes the contour lines come alive (e.g., topographic maps are great, but a 3D satellite image of a mountain peak can be better).
If you are so motivated, find Mt. Washington in Oregon on GAIA and you will see there is one 5 mile trail in and out with 3000’ of elevation gain. Looking at the same mountain on Google Earth enables you to see the steep terrain and lack of vegetation near the peak.
Power Considerations: From the environmental conditions that were passed, I knew we were going to be operating with relatively narrow margins.
In my opinion, one of the first steps in the process of considering a high-altitude hoist is identifying fuel options.
Using ForeFlight, Redmond Airport is 30 miles away from Mt. Washington, providing the option for the helicopter to get light. I have been involved in missions with similar environmental conditions, but we were unable to prosecute them because fuel was much farther away. With longer transit distances to fuel, pilots do not have the ability to get light enough to hoist.
Pro Tips: The NWS Cloud image is a great tool available in ForeFlight imagery, which depicts cloud bases and tops in MSL. A quick glance at the NWS Northwest Cloud image indicated that it was SKC in central Oregon. That said, big mountains often create their own clouds. Forecast products may say SKC but around the peak, there can be a pesky cloud that goes down to the surface. During daylight, ground parties on scene can provide general weather reports.
Checking the ForeFlight Redmond Airport page, we selected weather, then winds to see winds and temps. Winds and temps aloft showed that it was hot (13°C at 9000’, 19°C at 6000’) and winds were calm (9 kts at 9000’, 3 kts at 6000’).
Work high to low on the lapse rate to avoid getting caught by a temperature inversion (e.g., 9000’ 13°C + 3°C for 7500’ = approximately 16°C at 7500’).
For quick situational awareness, put your finger on the location of one of the large Cascade Range Mountains in ForeFlight, then in the upper right, go to WX. Foreflight will provide you with density altitude (“DA”). DA was around 10,000’ (for anticipating weather away from airports, this technique works well for a ballpark DA – it is not as effective as the National Weather Service Graphical Forecast for Aviation -GFA- for ceilings and vis, specifically fog).
Power Calculations: My pre-mission performance planning starts with the cruise charts. On one chart, pilots can compare power available to power required at different weights. With altitude and temp, pilots refer to the cruise charts for some quick data. Open your flight manual and reference the cruise chart. On scene conditions were estimated to be 7500’ and 16°C. Using the 8000’ and 20°C cruise chart, we obtained a lot of critical information within a few seconds. The previous post, “Expedient Pre-Launch Planning for Time-Sensitive Power-Limited Missions”, details how to derive hover performance (HOGE) from the cruise chart.
These were the values for 1.0 Engines:
1. MTA about 99% (single engine MTA x 2)
2. ITA 96%
3. 30 min power 87% (ECS ON)
4. CTA 77% (ECS ON)
5. HOGE at 18,000 lbs. 94% (no wind)
6. HOGE at 17,500 lbs. about 91% (no wind)
If there were strong winds following the curve on the chart, intersecting the wind velocity (indicated airspeed axis) shows the HOGE benefit. That said, high winds in the mountains introduce other challenges (down drafts and turbulence). I will cover wind and terrain analysis on another day.
Taking another minute, we can double check our work in TAB data – at 7500’ (slightly lower then above) and 15°C (slightly cooler then above):
1. ITA was 99%, which TAB data says our helicopter can HOGE at 18,800 (***remember this is the ITA value which only provides a 2-3% margin before the helicopter droop turns***)
2. To get to that 10% margin from MTA, we needed to turn on the contingency power and be about 1000 lbs. lighter (1000 lbs.= 6%).
3. Hovering at 17,800’, no wind would require about 93% (***remember this rule of thumb can get you in trouble at altitude… however, this is a good double check of the data derived from the cruise charts to ensure we didn’t make errors…see previous post on Tab Data***).
Fuel Management: At altitude, time on scene will be limited due to performance margins. The crew on the Mt. Washington case departed with a standard unit fuel load of 4200 lbs. and arrived on scene with 2800 lbs. Although too much fuel to hover with a 10% margin, power margins were acceptable for maneuvering around the terrain until locating the survivor.
Knowing that our unit’s zero fuel weight with a normal SAR load was roughly 16,000 lbs., my broad brush assessment was that we needed to hoist with about 1800 lbs. of fuel. 30 miles to Redmond with SKC allowed the crew about 45 minutes on scene before a bingo of 900 lbs. (300 lbs. to get to Redmond where the crew would land with 600 lbs. – more time on scene if we were able to deploy the swimmer, orbit at max endurance, then recover). Provided the short distance to Redmond, excellent weather, and the gain, I considered a 600 lb. bingo conservative, but due to the reported condition of the survivor, a lower bingo may have been warranted on scene. Additionally, if the crew could not package and recover the survivor in that time, the helicopter could leave the swimmer on scene, refuel at Redmond, and return for the recovery.
Because we regularly fly long range missions offshore, our community can be spring loaded to assume that more fuel equates to less risk. In mountain rescue, we need to be cognizant of this community bias. Where fuel endurance needs to be balanced with power margins.
Actual Power Requirements and Left Pedal: The survivor was located and the crew elected not to dump fuel. They established a hover into the wind, away from terrain, with a good escape. As they slowed to zero ground speed, the left pedal hit its stop and a slow right yaw ensued. The pilot at the controls continued to apply full left pedal and smoothly accelerated to forward flight, following the right yaw with cyclic (see the “Left Pedal Awareness and LTE” post). They jettisoned fuel and returned for another power required check, with a margin of greater than 10 percent. The crew noted ample pedal authority and executed a direct deployment and quick strop recovery using some vertical surface hoist techniques, likely saving a life, and preventing a ground party from commencing what would have been an exceptionally dangerous, and potentially impossible, evacuation.