High DA

March 28, 2012 by Tim McAdams

According to the NTSB, on the morning of August 7, 1998, after flying a Hughes 269B helicopter about 30 minutes herding cattle, the pilot located three cattle that were in a gated adjoining pasture. He landed the helicopter in a large mesquite flat near the gate so his passenger could get out and open it. After positioning the helicopter, he attempted a confined area takeoff. He stated that he had to lift straight up to clear trees. Upon reaching a hover just above the treetops, the pilot felt power was bleeding off so he lowered the nose trying to get airspeed. Unable to reach effective translational lift he turned toward a narrow clearing using right pedal and reduced collective to make a run-on landing. Upon ground contact, the right skid dug into the rain soaked ground, and the helicopter rolled onto its side. The commercial pilot and passenger were not injured.

After the accident, the pilot reported to an FAA inspector that it had been raining for a day and a half prior to the accident and that the weather was hot and muggy. He estimated the temperature to be about 95 degrees with high humidity and no wind. He also stated that he did not believe he had any type of mechanical failure and that the engine seemed to be performing normally. He felt that the density altitude, gross weight and out-of-ground effect operation all contributed to the accident.

Helicopter performance is a function of the density of the surrounding air. Density altitude is the reference standard used to measure performance and is determined by correcting pressure altitude for temperature. What is normally not factored into performance charts is the amount of water vapor present. Known as relative humidity, it is the amount of water vapor present (expressed as a percentage) versus the amount of water vapor the air can hold for a given temperature. Water is comprised of hydrogen and oxygen, which is less dense than the oxygen and nitrogen that make up dry air. As the humidity rises, the water vapor displaces the air molecules and lowers the density. Cooler air cannot hold a significant volume of water vapor, however hot air can hold a large amount, so as temperature and humidity rise aircraft performance will decrease.

Charts in the flight manual can be used to predict aircraft performance for a given density altitude. Since they are typically for dry air conditions, when temperature and humidity are high it becomes important to reduce expected performance levels. It is not just airfoils that are affected by humidity, but engine performance as well. A combustion engine can lose as much as 12 percent of its power on hot and humid days versus around 3 percent for a turbine.

  • Avi Weiss

    I’ll get on my soap box again for better instrumentation, especially as cockpit electronics continue to shrink in size and price (relatively), but increase in performance.

    For years, only “high-end” piston aircraft had any form of real-time “engine monitoring” to monitor ACTUAL power being produced. Now that value is readily obtained from JPI/GEM engine monitoring.

    Among the many duties a pilot performs, aircraft energy, fuel, and power management are the MOST prevalent and critical ones (AVIATE, navigate, communicate), and given the numer of accidents that seem to occur that have direct causal association to LACK of awareness / quantified aspect of aircraft energy, fuel, and power available, it would make sense to me to aval pilots to such instruments that would provide that information in a simple and real-time fashion: Doing airwork with load? how about a green-yellow-red indicator to show whether auto is possible/safe from current energy profile (altitude, speed, temp, etc). Can I make it over these trees? How about an indication of AVAILABLE power?

    Yes, yes I know…pilots should be able to determine all this, it’s too expensive, there’s no product/market, my dog ate my homework… The excuses are plentiful. Let’s stop making excuses and start making instruments that can provide the necessary info to help pilot judgement.

  • Jeff Bobelak


    In a helicopter an indication of available power is how much MAP it takes to hover IGE. In the Schweizer if I remember correctly you check for maximum power while in cruise flight by pulling maximum pitch/airspeed, maintaining altitude and noting MAP. Lastly density altitude calculators are widely available on the internet that include dew point into their calculation though the factory manuals – which I believe is the point of the article – are sadly lacking any kind of indication how how to adjust for humidity.

    Taking IGE MAP readings and doing a proper power check in accordance with manufacturer’s instructions should be a no brainer for a commercial pilot. I do agree though that some kind of flight computer that would give you some kind of value would definitively reduce pilot workload. I maintain that real time engine monitoring gauges indicating available power are already in place and a properly trained commercial pilot should have no issues with interpreting them.

  • Alan Barnes

    “In a helicopter an indication of available power is how much MAP it takes to hover IGE. In the Schweizer if I remember correctly you check for maximum power while in cruise flight by pulling maximum pitch/airspeed, maintaining altitude and noting MAP.”

    Huh? The first sentence just isn’t correct. And the second makes no sense to me. “pulling maximum pitch/airspeed?” What does that even mean? Surely you don’t mean just yank the collective or fly the aircraft right at VNE and see what you’re using for power. And perform a “power check” in accordance with the manufacturer’s instructions? Good luck with that!

    Like most of the incidents reported here so far, this too boils down to PILOT ERROR. He *estimated* the temperature. He was flying at a high gross weight, high temperature, and out of ground effect. I’m *guessing* that he never bothered to check the PA, the actual temperature, or consult the DA chart.

    Another thing to keep in mind is that performance charts are generally created using new helicopters, shiny blades, zero wind, an experienced test pilot, etc. If the manual says OGE is 2000′, your helicopter may not be able to hover OGE at that altitude. Or even 1000′.

    This didn’t happen because of a lack of gauges or indicators. This happened because a pilot simply didn’t do his job and exceeded the capabilities of the aircraft. The NTSB agreed. Their finding?

    “The pilot’s failure to maintain control during a vertical, out of ground effect takeoff which resulted in a hard emergency landing. Factors were; the exceeded hover performance of the helicopter, the high density altitude, and the lack of suitable terrain for the emergency landing [high obstructions (trees) and soft ground]. ”

    The good thing to come out of this is that he reacted properly. Had he been able to get a suitable landing spot (e.g. a non-waterlogged field) everything may have turned out OK. And nobody was injured. If he’s still flying, I bet he checks those DA charts on hot days now!

  • Alan Barnes

    and, for what it’s worth, the NTSB brief for this accident is http://www.ntsb.gov/aviationquery/brief.aspx?ev_id=20001211X10850&key=1 and the full narrative is http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20001211X10850&ntsbno=FTW98LA356&akey=1

    Maybe Tim could start including this with posts that deal with incidents? *hint* *hint*

  • Jeff Bobelak

    We seem to be in agreement with everything but power checks.

    When you pull up to an In Ground Effect hover you make note of the power or Manifold Absolute Pressure reading – that is used for a couple of things.

    I admit the factory procedure is weird in a Schweizer and I don’t have every step committed to memory but yes that it roughly what you do – see what the MAP reading is while in cruise flight while maintaining rotor RPM and altitude. Trust me, you will not get any where near VNE before you start bleeding off RRPM or altitude while pulling as much pitch as possible. Again, please excuse any mistakes made that as I don’t have the exact steps committed to memory but you have the gist of it.

    The difference between those two MAP readings is the available power.

    Not sure why you would have to have “good luck” to perform a power check in accordance with manufacturer’s instructions. I outlined the Schweizer procedure to best of my memory, I have never flown a Robinson so I couldn’t begin to tell you how to do it in that and here is a good link showing procedure in a Bell 206 : http://helicopterforum.verticalreference.com/index.php?app=core&module=attach&section=attach&attach_id=2522

    In some instances I’ve seen where checklists have said to make note of the MAP reading before starting the engine and using that to calculate available power.

    The factory checklist for the Schweizer says to use 1″ more MAP than hover power to climb out.

    I’m not really sure why basic private pilot 101 power checks don’t make sense to you, there’s a couple of reasons right there to make note of IGE hover power.

  • Paul Goldasich

    I trained in the Robinson R-22 as has many, but made a transition to the Schweizer. I also have time in the Bell 206, Bell 47-G and am currently flying the Robinson R-44 II. It really doesn’t take much to realize significant temp and humidity especially while doing your pre flight, and this should throw up a red flag for any pilot. This would be the time to find out exactly how hot it is and how humid it is, figure that it will probably get worse as the day goes on, and plan to adjust your flight regime accordingly. You would want to fly more conservatively which means controlling the weight issues and staying away from flight regimes that would require very high power settings.. Since the pilot and/passenger can’t go on a crash diet and lose a significant amout of weight prior to the flight, the only weight that can be adjusted is by the fuel quantity on board, and/or any other cargo that may have been on board. I know there is not really any room to carry stuff in the aircraft, but there are baskets made that mount on the skid assemblies to carry cargo. This weight can be managed according to known conditions. It may mean that the pilot will have to make more frequent fuel stops, but he should be able to manage that, and with it being that hot, he would probably need something cool to drink anyway to keep hydrated. It all comes down to some pre flight planning and common sense. FLY SAFE.

  • Dennis Vied

    Oh, c’mon, guys, you’re arguing over nothing. Are you just trying to show how much you know?

    Avi’s comments are right on the mark. The new glass cockpits have been responsible for a great leap in situational awareness. Sure, you can compute all that stuff in your head, but it takes time, and your brain is fallible. All he’s saying is that we need the same kind of tools to make it possible to get that stuff instantly without having to compute it yourself. It would be a great aid to situational awareness. It doesn’t mean you don’t know how to compute it yourself, and you’re always going to run a mental check on the computers occasionally to make sure they’re telling you the truth, but it’s the way we need to go.

    So, go to your rooms and stop this bickering.

  • Dan Colburn

    The price of beef just went up!

  • Avi Weiss

    Thanks Dennis. I thought I was losing my touch for a minute there. Glad my point was not lost on everyone.

    Jeff, Allen: As I mentioned in my original post, but will make much more explicitly here: Any competent helicopter pilot with a modicum of experience in a particular aircraft (make, model, and perhaps S/N) and basic airmanship and judgement skills should be able to complete temperature and performance analysis with current methods to determine if an aircraft will perform as necessary in any given combination of conditions (external and internal to aircraft), any be sensitive enough to cross-check that analysis with what is actually occurring.

    But the same can be said for almost every instrument that has been added to the cockpit. The value of any instrument isn’t necessarily to provide information to a pilot he couldn’t obtain for himself some other way, but to provide that information CONVENIENTLY and CONTINUOUSLY.

    To put a finer spin on Dennis’s statement of my point, conditions change, and pilots get saturated, and “available bandwidth” (time and brain power) may dwindle or error may creep in. Continuously determining or even remembering to “appropriately” determine the necessary info sometimes can be overlooked in the heat of effort. Having instrumentation that can provide a CONTINUOUS and ERROR-FREE source of critical aircraft energy state in a SINGULAR easy-to-process display may help those pilots who make that error, and likely KEEP pilots from getting in trouble as a result of it.

    Of course like any instrument, like say FUEL, no matter much effort is placed into “idiot-proofing” the decision-making process, someone comes along and invents a better idiot, so nothing will prevent ALL pilot erros and accidents, but providing more meaningful, timely, context-specific, readily-digestible information to pilots when they need it most can make the difference between a successful flight, and one less so.

    Hope that clears it up a bit.

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