This is a continuation of the "How's the Weather" sub forum. Didn't want to hijack the thread.

I think I am getting grubered. To get this clear in my mind.........let me ask a few questions.

What is the highest elevation....and runway lengths....of heavy passenger planes?

In other words......what is the runway length and air density ratio?

And are runway lengths.......that close to that ratio?

I believe the flights are cancelled do to runway aircraft air conditioning capacity.

Not all flights are cancelled. I have seen the news reports about air density.

I think it's airline PR grubering.

 

You are referring to Density Altitude when it comes to airplanes. DA is the altitude in a standard column of air at which the current air density (pressure) is found or another way to think of it is DA is the altitude that the airplane thinks it is at from a performance standpoint. Stand on the ground and DA will increase as the day goes on and temperature increases.

DA is computed using the actual standard altitude corrected for barometric pressure, temperature and to a lesser extent, humidity. As the altitude increases, performance drops off due to less mass air over the wings and in many planes, engine performance. The pilot's operating handbook for a given aircraft will have charts and other data by which the pilot can predict the performance of the airplane in various flight modes, takeoff and landing distances, climb performance, engine-out performance etc.

Required runway length increases with DA because to get the same mass of air over the wings with less dense air you have to fly faster. That means increased takeoff rolls to accelerate to that higher speed and longer landing rolls because you come in faster. The distance required to accelerate to takeoff speed then shut it down and stop in an emergency is also important in commercial operations. Sometimes on a hot/high day, you have enough runway to take off but not to handle an abort. Airline pilots compute takeoff, accelerate-stop and landing distances for every flight.

Large aircraft have more trade-offs. You can fly on a cool day from a sea level airport with a bigger payload than on a hot day at a high airport. You usually trade off fuel for payload and vice versa. In the case of an airliner, there are specific performance requirements that have to be met and sometimes these can't happen at a hot/high/humid airport. Sometimes, you have the performance with the payload but inclement weather at the destination will trigger the requirement for additional fuel for diversions, missed approaches etc. In that case, the flight may take off but with fewer passengers or off-loaded freight - usually passengers get left behind.

The cancelled Phoenix flights were run by RJ's which have lower performance than something like a 737, 757 etc so they will be the first to feel the pinch. By the time they're light enough to deal with a high DA, its not worth flying. The aircraft in Phoenix had a 118degF limit cited. I don't know specifically why but it could be that that's where the data ends in the POH. It's not smart to fly outside the known limits and for 121 carriers, probably illegal.

There is no hidden PR grubering, whatever that is, nor other nefarious agenda. The pilots want to fly as much as the passengers but safely, within limits and with reserve performance. The guys who build, test and fly the planes actually know what they are doing and any legitimate pilot will err on the side of safety.

https://en.wikipedia.org/wiki/Density_altitude

 

So airplanes use all the runway at full power normally?

And even if they don't........a 10 degree temp rise......can make our runways useless?

I understand and agree with all of the response/reason principles. I just don't believe that is the real reason for the THESE cancellations.

Again, I understand the principle. If I had a plane on a farm......and I made my runway at the height of summer......and had to use full power and took all the runway to take off normally.........Then a heatwave comes thru.........I can't take off. I got that.

But airports are not built that way. And airports are multi-length and multi-directional.

An increase in velocity can easily overcome the change in density. If the engines can not overcome the density change of 10 degrees.............we are way under engineered.

My bet is on the air conditioners.

I was not aware of abort distance......which would double the length needed. So we are definitely under engineered.

 

Most airplanes use full power at takeoff. Jets and turbo/supercharged pistons have frequently have reserve power that isn't necessarily used on takeoff.
You don't use ALL the runway - usually , you make sure you have enough to take off/land under the current conditions. I didn't mention wind but that's a factor too.

The runway directions for airports are aligned to the most common prevailing winds in the area. Crossing runways are common and usable if the crosswinds are within limits. After a weather front passes, winds change directions and planes use the other direction of the runway.

10 degrees?? Aircraft perform at hot temps all the way down to seriously sub-zero temps, on the ground and in the air. Each plane is designed for its mission and performs within a set of limits. Outside of those params, get a different plane or wait for conditions to change.

I don't know the actual reason for the cancellations and your A/C guess is as good as mine but passenger comfort is not really too high on most airlines' list of priorities. I've been in and out of PNX in the summer and it is pretty brutal.

 

10-4........the hottest I ever seen was 118 F in Texas in the 90s. At 122 F.....I would not leave the house. I wouldn't want to get into a car at 122.

Edit: not 10 degrees........10 degrees in change.

 

Yeah, hot days in the TX panhandle can be challenging to light aircraft on a hot day, particularly when the air is convective. I used to go in and out of South Lake Tahoe a lot. The highest DA I've seen personally was about 11K. Considering most normally-aspirated light singles get sluggy around 6-8K, blasting off and trying to climb over the mountains to get to SFO wasn't something I liked very much and eventually quit doing. That runway is 8500 feet long and unwinds real quick in one of those.

 

I was rebuilding plastic extruders in Weatherford. It was 130 F in extruder bay. We would go in and work for 15 minutes.....come out and drink water for 15. It was a terrible job. From then on....tried to avoid Texas summer jobs.

 

I was rebuilding plastic extruders in Weatherford. It was 130 F in extruder bay. We would go in and work for 15 minutes.

An attic, in summer, in Florida, was measured at 135F. My limit is 20 minutes before brain fog starts. (I should have quit at 15 minutes.)

Back on the subject matter:
What I see here is that landing gear has nothing to do with limiting take-off or landing speeds. If it did, @JohnInTX would have mentioned it.

 

What I see here is that landing gear has nothing to do with limiting take-off or landing speeds. If it did, @JohnInTX would have mentioned it.

I wasn't thinking that deeply... typical but I would guess that it is the brakes more than the wheels/tires that is important. As I noted in the linked thread, one important consideration is the aborted takeoff and, I suppose, the short landing. Airplanes are designed for minimum, and I mean minimum, weight so the size of the disc/rotor/caliper is limited to ground maneuvering, not so much jamming on the brakes to stop after landing or after an aborted takeoff. A typical landing profile uses minimum brakes, airliners use reverse thrust, piston hacks like me just let it roll out.

I was building a kitplane before the FAA freaked out about some medical issues and did considerable research on braking systems (my RV7A uses differential braking for ground steering) and found that the standard brakes were OK for basic handling but cramming my tuchus into it on a hot day at gross weight and aborting a full speed takeoff was problematic kinetics-wise. I bought bigger brakes which meant bigger wheels, tires and more weight to lug around. And that is the problem / tradeoff in aircraft design - do you lug around an extra few pounds even when you don't need them for most cases or do you accept the limits of a smaller system and maybe just say you don't take off on a hot / high day with a full load on the off chance that you might have to shut it down, stand on the pedals and hope you don't go through the fence or have a fire. In a kitplane its up to the pilot but in an airline operation, all of that has been thought out and codified beforehand. Takeoff performance fits under the graph - we go. It doesn't, we don't.

It's science and physics but also procedural and legal. As I said over there, airplanes are tested to design limits plus a bit and those limits are documented. Outside of the box and you are the test pilot. Not all of those excursions end on a happy note. If the RJ would fly at a documented 118F, it will fly at 120F but if there is any problem with the flight, the pilots will be toast leagally-speaking. That's why they cancelled the flights IMO.

 

@#12 Here is a page from a Bombardier CRJ manual showing the max ambient temperature approved for takeoff/landing as ISA + 35 degC. ISA is 15degC at sea level so that is 50degC at sea level (122F). Add in the standard lapse rate 3.5degC/1000 feet for the airport's 1135' elevation and you get pretty close to 118degF. That's what Bombardier has demonstrated with respect to the required performance to take off, climb, obstacle clearance, deal with engine out etc. The engineers slip-sticks told them that is what it was so that's what it is.

 

That is quite interesting. I had no idea the limits were so close to the operation. I think I'll stay out of planes.

Balloons aren't as boring as they look are they?

 

I have a friend that's a mechanical engineer and has worked in the medical implant industry. He was appalled at the quality of talent in that industry, and the low quality of the products. He tells me that the best engineers all end up in aerospace where there is an emphasis on knowledge and quality. Other industries get the rest. He also worked at Chrysler and that was the worst he's seen.

 

that is not good to hear.....for us older folks. we don't like that word medical.

 

I have a friend that's a mechanical engineer and has worked in the medical implant industry. He was appalled at the quality of talent in that industry, and the low quality of the products. He tells me that the best engineers all end up in aerospace where there is an emphasis on knowledge and quality. Other industries get the rest. He also worked at Chrysler and that was the worst he's seen.

I have a friend that worked his whole life at GM. He avoids risk at all costs. They beat it into him. I'm surprised we ever moved forward from the Model A. Oh wait.....

 

Thank's JohnInTX........I believe I have it now. Appreciate the links.