At 14,427 feet Daochen Yading airport is the highest commercial airport in the world. I only recently saw that number and was a bit stunned. That’s higher than Pike’s Peak in Colorado! edit: Or rather Pikes Peak, spelling counts @mrmrman thanks :)
Can aircraft really operate commercially from such an altitude I wondered.
Anyway I wanted to compare take-off IAS’s with the same aircraft type at MTOW but only make changes to altitude, temperature and wind.
I was comparing Denver with Daocheng Yading, which is getting up toward 3 times higher in altitude.
Should I have been surprised that there was some difference in IAS at the same aircraft weight? I needed higher IAS at Doacheng Yading.
Is it possible the higher IAS to get airborne at Doacheng Yading is due to lower thrust as altitude increases (to be honest I gunned it at 100% throttle).
What I mean by the effect of lower thrust:
Would lower thrust at altitude cause less vertical direction of thrust supplementing lift when you achieve the takeoff rotation angle?
So maybe you need a bit more IAS because the lift is less supplemented by the vertical direction of thrust.
As the highest airport in the world, only the A319 Plateau is currently approved by CAAC to operate at Daocheng Yading. In practice, take-off and landing speeds(IAS) at Daocheng Yading don’t differ significantly from those on the plains. However, the increased altitude and consequent reduction in engine performance means that take-off loads are strictly limited. Specific data can be generated by Airbus PEP and analysed by specialists. It is also because of its high altitude that the SID procedure makes rare use of the RNP1-based RF procedure, which allows the aircraft to control the turn gradient and thus obtain better climb performance.
Yes, I’ve flown here a couple times in game and had no major issues. However, when landing I recommend you to use a higher reverse thrust setting (50% - 70% and idle reverse at 80 knots) and medium autobrake. Aircraft performance is severely impaired at ZUDC.
Maybe I was wrong about the difference in IAS necessary for positive climb. I thought it would be the same, so perhaps my technique was off.
I just tried again, comparing takeoff IAS at SFO vs ZUDC
For test purposes, 7474 at MTOW (don’t try this at home).
SFO is basically at an altitude of 0.
ZUDC at 14,427 feet (as high as the highest peaks in Colorado)
Conditions:
SFO temp of 15C and no wind
ZUDC temp of 40C and a 30kts tail wind (yes, that’s tail rather than head wind)
This time it seemed IAS for start of positive climb was about the same. It’s a bit easy to misjudge because everything is happening so fast at rotation.
What’s clear though is that although IAS was about the same, GS of course was hugely different. At SFO GS=IAS; at ZUDC GS was much higher (even without the tail wind, lift off was well in excess of 200kts GS).
The temperature alone is a huge variable there. If you want to make this test really representative I’d go into solo mode and make the conditions the same. 15 c is standard temp so it doesn’t affect density altitude but 40 c is pushing your density altitude to more than 20,000 feet at the other airport. (assuming standard pressure) Not to mention the tailwind which will have a huge impact on roll distance.
Also IAS should be the same if I’m not mistaken. I don’t know how this is all modeled in IF, but speaking from the real world takeoff speed in IAS should not be affected by density altitude. Since indicated air speed measures the amount of dynamic pressure on the pitot tube and lift is generated by dynamic pressure it should all be the same. The same thing goes for other speeds like stall speed, you will always be using the same indicated airspeed for those things, it’s when you start using true airspeed, ground speed, etc that they need corrected for other factors.
In any case the ISA (International Standard Atmosphere) used in the performance analysis has a different standard temperature at different altitudes, the simple formula for calculating the ISA temperature is 15 - 2 X [height in feet / 1000]. This formula is only applicable below the top of the troposphere.
So in terms of ISA, all highland airports are hot airports!
However, I would think that the IAS wouldn’t change rather than the GS. For example, a takeoff from LFPG which is 300ft MSL would require something between 140-150kts IAS to takeoff and with no particular wind you would takeoff at around the same speed GS, 140-150kts.
HOWEVER, if you take the example of Mexico city, for a takeoff at 140-150 IAS, the aircraft would only lift off at around 160-170kts GS, due to the altitude. This is something that is very obvious when flying IRL you really realise how fast you are touching down on the runway when landing in MEX.
I would say that for similar temperature (≈15-20°c), similar wind (≈0kts) the takeoff IAS at ZUDC would stay at around 140-150kts IAS but the GS would be significantly larger. What I’m trying to say is that it is 100% normal to experience VERY high GS.
I would like a pilot to confirm this though because I’m making assumptions based on my personal experience flying is these airports.
I’ll probably edit this message after trying in ZUDC.
The first thing I was messing around with was trying to demonstrate this for the wildly different environmental situations. And I thought I was getting a bit of difference. But I can’t rule out my technique was not maybe creating the apparent difference - I was trying to consistently rotate to be at a specific pitch target while reaching specific IAS target.
I was thinking, you get a set amount of lift for an IAS combined with an AoA.
You lift off when that lift just exceeds weight.
But if you pitch up to, say, 8 degrees your engine thrust is no longer horizontal.
I made some thrust and weight assumptions and calculated a vertical component of thrust to be as much as 5% of aircraft weight (actually rounded up from 4.6%).
I have no idea if the thrust would be cut in half at 14,000 feet compared to sea level. But changing that 5% supplement to lift changes IAS for a given AoA when trying to equal weight.
Splitting hairs I know. And maybe too small to notice?
Ground speed at takeoff was as massive at 14,000 feet as the acceleration was snail paced. The effects of density altitude squared!
By this do you mean the OAT at a high airport may not seem hot, but hot has to be judged against the formula temp for that altitude as far as whether take off performance is negatively affected?
That’s an interesting example of how easy it is to forget if you’re flying towards the limits of high, heavy and hot, you run out of capacity to turn.
That is an interesting theory about the thrust, and depending on how Infinite flight models it definitely non negligible. I’d say probably a soft field type technique would be best to see IAS alone, but that would also be most susceptible to the engine performance difference. That is something of a conundrum to prove this point practically 🤔
Top: Takeoff from ZUDC, 14471ft. No wind, OAT 15°C.
Bottom: Takeoff from ZBAA, 115ft. No wind, OAT 15°C.
Both were taken on the A319-100 at about 100 knots IAS.
This shows how your ground speed increases with altitude at a constant IAS due to air density and how IAS is calculated as what is sensed by the pitot static system.
For the difference in GS at different altitudes, there is actually a specific formula for flight procedure design to obtain GS from the IAS, which you can obtain from the ICAO document Doc.8168
This is a table of the take-off performance of the A319-115 aircraft at the world’s third highest airport, which can be used as a reference. I will generate a performance chart for ZUDC in a while.
