Tuesday, June 20, 2017

Oh Noes! It's So Hot Airplanes Can't Fly!

This was the headline about life in Phoenix yesterday and into today.  Well, except for the "Oh Noes!" part.  How hot was it?  118, or 47.8 C.  (Standard joke:  Arizona person, "but it's a dry heat".  Me: "so's a Bessemer furnace")  Seems to me that 118 isn't extremely hot - I'd swear I see temps like that all summer long looking at TV weather maps - and it turns out it seems well off the all-time high in Phoenix of 122. Certainly not a record, there.

So what's going on here?

The only flights that were cancelled were flights of a Bombardier regional jet.  The articles didn't mention the specific model; they just called them a CRJ and there are three models in that series: the CRJ700, 900 and 1000 (pdf warning).  The company I retired from, whom I've always protected from my ramblings by simply referring to them as Major Avionics Corporation, was a top tier supplier to Bombardier and they were, in turn, one of our largest customers.  Just as I can tell you that I did the RF design on a handful of radios on those airplanes, I can tell you that no electronics system we ever sold them was rated to less than 75C, and everything I tested was tested at 85C (167 and 185F, respectively).  118 is pretty meager compared to those.

The answer lies in the airframe and power.  Aircraft are rated for a certain maximum takeoff weight and that must be what the wing area can lift given the speed available out of the engines.  The catch is that physics says lift depends on the density of the air the aircraft is operating in and that's rarely the same as it was designed for.  Pilots refer to density altitude; the equivalent altitude of an airport based on temperature (and humidity) compared to the standard conditions the aircraft was designed for.  At higher temperatures, air has a lower density - it's thinner; fewer molecules are going over the wing.  That lower air density reduces how much lift is generated on the aircraft's wings and it reduces engine efficiency; a double whammy.
On a hot and humid day, the aircraft will accelerate more slowly down the runway, will need to move faster to attain the same lift, and will climb more slowly. The less dense the air, the less lift, the more lackluster the climb, and the longer the distance needed for takeoff and landing.
The CRJ aircraft simply aren't capable of 100% operation in their service area.  They'd need more wing area or more power out of the engines to operate every day.  I understand that aircraft design is full of compromises; heck, all engineering is, and that rating the plane to take off when the density altitude is above the current limit may not have been possible without major changes to something they were primarily designing for (probably cost per seat mile).  It's pretty common in engineering that the last couple of percent of improvement in performance cost more than the first 80 or 90%.  I also note that takeoff air temperature isn't specified in the .pdf brochure I linked to above, so it's possible airlines expect their planes to not be available 100% of the time.

You might wonder if this is really unique, and if not, why it's national news.  I'd say it's not unique and yesterday was certainly not the first time.  Conde Nast Traveler reported on the same situation one year ago today, and that article pointed out it's not that unusual.  It seems really hot days in the third week of June aren't unusual in Phoenix.
This is not the first time airplanes have felt the heat in certain parts of the U.S.—last summer, some planes faced similar challenges, and in past decades, we've seen the same story play out: In June of 1990, temperatures hit 120 degrees, so hot that the asphalt on the tarmac softened, aircraft couldn’t move, and they had to ground flights.
Since it's not unique or the first time, then why is it national news?  Could it be the media is trying to tie hot weather to climate change, and tie that to Trump announcing we're getting out of the Paris climate accords?   That's a guess.  I'll report on stories because I find them interesting.  I don't think the media works that way.

Conde Nast Traveler - Getty Images


  1. why is it national news? Could it be the media is trying to tie hot weather to climate change, and tie that to Trump announcing we're getting out of the Paris climate accords?

    Exactly. It's not genuinely made-up fake news, but it's this way every summer when the temps hit that magic number and as you point out, it has to do with air density. It is a 'dry heat' and that's why they ground them...or in this case, "it". Corrupt, elite, smug, nasty, progressive, lying mainstream media outlets have an agenda and only broadcast stories that can fit within that particular bias. So it is here with the grounding of a class of Bombardier jet that requires a bit more air density (humidity/lower temps, etc.)

  2. They could offload pax or fuel to make the reduced max take-off weight, but the fuel is already limited to min cost for the required leg so it's offloading pax and we all know how that goes!

    1. They simply needed to make a cabin announcement that one of the Trumps would be on the flight, and every liberal, snowflake, and Democrat (but I am getting redundant here) would run screaming off the plane, looking for a "safe place" - or at least somewhere to change their linen. At that point, a stiff breeze would loft that puppy airborne.

  3. When you have 3 major networks all with news programs plus Fox/CNN/MSNBC/BBC etc running "news" 24/7 unless something
    big is happening there isn't enough real news to fill air time.
    So on slow news days air time is filled with fluff and nonsense.
    The same occurs in print and internet news sources. Nothing in
    broadcasting is abhorred as much as "dead air" so every minute
    MUST be filled with something......anything.

  4. But if they really wanted to increase their viewership, Dan, they could run wall-to-wall HD Rule 5:

  5. Careful, Mark. The media's notion of Rule 5 might be Michelle pole-dancing around the Washington Monument. Or worse, Mitch McConnell dressed as Wonder Woman, pole-dancing on an elementary school playground. That would attract the Pizzagate elite, as well as the rest of NAMBLA.

  6. Re: (Standard joke: Arizona person, "but it's a dry heat". Me: "so's a Bessemer furnace")

    Having lived in both Florida and Arizona during the summer months (and in Phoenix on that day in June 1990), I much prefer AZ's "dry" heat at 120 over FL's "not-dry" heat at 95. But then, it's a personal preference. And here it is getting on July and I'm looking forward to leaving Tidewater's mid-90s - temp >and< humidity - in a few weeks for the Southwest's 100s ... just for that dry heat. YMMV of course. :)

    Flying: Cuzco, Peru (11,000+ ft) is an interesting place to land or take-off.

    Climate change - bah, humbug. If it weren't for climate change, half of North America would be covered in ice. Personally, I have more belief in solar radiation variations and volcanic activity than soccer moms driving SUVs. Perhaps cause and effect are backwards - CO2 increase due to warming rather than causing it.

    Trump: He may be a great president; he may be a not-great president. The Democrats - and mainline Republicans - won't give him a chance to really screw up. In either case, I'm thankful the President isn't the alternative.

    Great post and comments. So many topics in one.

    1. I've been in Phoenix in late August and my wife and I both thought it pretty much felt like here. Instead of 90 with a heat index of 105 here, it was more like 115 with a heat index of 105 there. Either way a nasty day to stay outside.

  7. News is a business. People buy headlines. Flashy headline generates income. It's that simple.

  8. You think you're getting buried in news coverage about this, living outside the blast zone? Try living here where we have local stations trying to fill 10 hours or more of scheduled news broadcasts EVERY DAY!

    I've lived here 60 years and these stories are nothing new. It's not so much that the planes CAN'T fly; it's usually because the tech manuals for the specific type aircraft don't SAY how the plane will respond at 120-125 degree temps. You don't want to try and fail with a load of passengers. (And remember: the temps at ground level on a concrete runway are higher still.) Back when we hit 122, the "big news" was that 737's wouldn't fly. Their manuals have since been updated.

    Solution: stay in the air conditioning, drink plenty of fluids, hang by the pool and pray for November!

    1. An important point! What the manuals and rules are saying is that fully loaded, fully fueled and with every worst case condition put together, it couldn't take off in the required distance. They weren't saying that on that day those specific planes couldn't take off.

      It is all done with an abundance of caution. As someone said, "I've known a lot of pilots in my life and I've never known one that would take off if they really thought they were going to buy the farm".

  9. You are right, but also very very wrong. No airplane can ever fly with full Fuel and full passengers and every possible problem anticipated.

    The manuals and performance charts tell the pilot how much fuel and payload can be carried and still meet the FAA required minimums for performance in all of the various flight envelopes. Those tables are complicated and difficult to put together, involving actual test flights in those weather conditions and allowing only very minimal extrapolations to cover gaps in the data set. They are put together one piece at a time for all of the various temperature Weight and Balance fuel Etc combinations possible.

    At some temperature, the manufacturer simply cannot continue testing and Gathering and supplying data. Where and ehen would you go to test your new airplanes performance in 125 degree temperature on a ten thousand foot Runway? I suspect that kind of testing could only be done one or two days a year, and maybe not every year, and only in a couple of places anywhere in the world.

    All of those planes would have been safe to fly, and I would have flown any of them with, say, 2/3 Fuel and a full load of passengers. Phoenix has Ultra long runways and is very safe. Certainly they would have been safer than every single day they ever took off out of Jackson Hole Wyoming or some of the runways in Central and South America at and near the 10,000 FT elevation level. But they were not legal to fly because the manuals didn't cover data above a certain temperature, and therefore there was no way to legally certify the airplane as safe to depart.

    I have to say, you took a more careful and detailed approach to the story than anybody else. Hope this is not seen as disparaging, just trying to fill in a hole that is probably important to maybe two or three hundred people in the US, all of us insufferable boors and not fun to take to a party.



    1. Thanks - you actually made me LOL.

      I was trying to emphasize the safety margin in all of those requirements. It's practically impossible to know the loading of an aircraft. Yes, they might have all seats filled, but are they filled with the design weight (I think it's 170 pounds) or a college football team? Is the luggage all underweight, or is it full of people going to the anvil collectors convention? Considering how important Center of Gravity is, I'm surprised airplanes sometimes don't end up tail down on the runway.

      Finally, if I may return the favor of being an "insufferable boor", when you say "... allowing only very minimal extrapolations to cover gaps in the data set." The right word is interpolations not extrapolations. Extrapolations are off the ends of the curve where there are no data points - it's risky. Interpolations are between data points and therefore less risky. Yeah, I can be too pedantic.

    2. I suspect the Venn diagram of all the people who care about my corrections, and then your correction to my correction, is probably about 27 people.

      And I suspect that between us we can name most of them.

      All best wishes.


  10. I agree with Mr Moynahan and would add that no airline is going to fly outside the parameters set out in the Airplane Flight Manual. To do so is illegal, creates very bad publicity and invalidates your insurance policy.

    I would guess that the reason only the CRJs were affected was that the CF34 engines that power them are flat-rated to 80F. 'Flat rating' refers to the line on a graph where the vertical axis is thrust and the horizontal is temperature. Starting at the left the thrust line is flat until you reach the rated temperature, at which the thrust line slopes downward. At 80F the engine is creating the rated thrust with the engine at full power, i.e. 100% RPM and at Max ITT. At any temperature below 80F rated thrust is created at less than 100% RPM and/or Max ITT. Above 80F the engine produces less than the rated thrust and there is no way to increase it, above 100% RPM centrifugal force could cause the turbine disc to burst with catastrophic results, while above Max ITT the high pressure turbine blades melt and the engine loses power.

    Less thrust means less acceleration which means you need either a bigger wing or a really long runway, or you have to start reducing the weight. After United's deplaning fiasco in April do you think any airline is going to go there again? "Yeah, we have to reduce the weight to takeoff safely so if all you fatsoes could get off the plane..." Could you imagine the s***storm on the internet? I believe the law allows the airlines to deplane excessive passengers when the flight is overbooked, I doubt that it allows deplaning because of high temperatures.

    In comparison to the CF34 the latest CFM56s are flat rated to 111F (up from 86F in the earlier versions).

    Another thing to remember is that the thrust of one engine must be sufficient to get the airplane airborne (over the mythical 50ft obstacle) once V1 has been achieved on the take-off roll. I believe that with a lightened load and a little headwind the CRJs could have taken off but for legal and liability reasons only a fool would have done so.


    1. Thanks for that clear exposition of "insider information" - information that I sure don't have.

      Another thing to remember is that the thrust of one engine must be sufficient to get the airplane airborne (over the mythical 50ft obstacle) once V1 has been achieved on the take-off roll.

      That's something that I should have commented about, because I'm at least aware of the requirement to be able to climb well enough on one engine to get into the pattern for a return to the runway. I believe that's really on half the engines, so that a four engine aircraft had to do it on two, right?