I didn’t find any thread on this topic but would be interested to discuss this.
There are several starting points when thinking about optimal altitude when flying into a headwind. For example, if you can go high into the flight levels, it looks like it’s still reasonable to climb up there because typically the wind does not increase as steep as does True Airspeed, when you are above the boundary layer where – in turn – wind is typically significantly less.
But if you don’t climb high (pick whatever reason), say below FL120, where should one go? Of course there are several things to be taken into account which may alter the picture, such as wind shear, clouds (where often wind shear occurs above) or mountains and so on.
But with a typical wind profile in Europe mainland, according to my experience, you often see about 5 to 15 knots of wind above surface and about 30 to 50 knots in FL100.
You have some boundary layer down low, so that the wind profile does not increase linearly to altitude. Same goes for TAS which does not increase linearly over altitude (as it is density dependent and this decreases more like asymptotic).
According to my own experience, and doing some flights where I climbed up to around FL100-120 and descended slowly and constantly down over about 40 minutes I found some sweet spot around 5000 ft AMSL, where TAS is still increased quite substantially over IAS, but wind profile often has not yet increased so much as it does in between FL50 and FL100. In my last flight into ~35kts headwind I had exactly the same ground speed in FL100 as I had very down low (say about 1000-1500 ft AMSL) , but had 10kts of GS more in FL50. (all done at same fuel flow, which converts best to similar power output)
Is this only by chance, or have others made similar experiences?
Edit: I know that apps like Skydemon put out the optimal altitude, but it practically always tells me to go as high as possible…which seems not to be all the truth available out there.
If you are talking about altitude for best fuel economy, you will be disappointed to find out that if you fly near the efficient MPG or efficient MPG*ASI then altitude does not matter much under zero wind (as you will be flying some optimal AngleOf Attack that converts height into distances), when it’s headwind, you just need to adjust that a bit by flying faster into headwind (to keep that efficient H/D conversion going on), when it’s tailwind, you slow down a bit to increase MPG, then how high you climb is irrelevant, I am too lazy to do Gliding or Carson speeds maths
If you are talking about best altitude for minium flight time, then the answer depends how do you fly: constant fuel flow? constant power? or constant rpm? or constant IAS? or constant TAS?
In the latter, whatever “your budget” is you just keep full power climb to a higher altitudes if TAS increase, if it decreases stop the climb and cruise
On NA SEP without oxygen, I doubt there is much to optimise for best TAS, in nil wind you fly FL80, if tailwind you fly FL120, if headwind you fly FL40
Ibra wrote:
constant fuel flow? constant power? or constant rpm? or constant IAS? or constant TAS?
In order to be able to compare it, I think constant Fuel Flow is the best number to fly. Of course there are some minor effects down low, such as it is not with the throttle wide open. But keeping the FF constant brings it down to a simple number, which strongly corresponds to power output (of course keeping all other settings constant, so same RPM and so on).
So @Ibra your number would be FL40 when flying into headwind…
UdoR wrote:
…I found some sweet spot around 5000 ft AMSL … Is this only by chance, or have others made similar experiences?
I’ve had the same experience several times, but there is a major risk of confirmation bias here.
The title reminded me of my days on flying C130s where particularly on the Atlantic you could encounter headwinds up to 200 Kts. We chose our route to minimise headwinds essentially by tacking across then to acheive a Minimum Time Track, initial we eyeballed it, but later a BA Computer coffed them out for us. The East coast of the US was the area where you couldn’t do much about the wind so you had to find a level where the effect of the wind was least. There is no magic formular to do this and no magic altitude. We simply calculated the lbs per mile for different altitudes and flew at the one that gave us less pounds of fuel per mile. It was a typical calculation a Navigator would be given in his yearly categorisation check.
SkyDemon will do this calculation for you based on prevailing winds(internet derived or manual) for your route…you do have to put accurate fuel burn data for climb, cruise and descent of course. It then gives you time and fuel burn at various levels, highlighting optimum.
UdoR wrote:
Is this only by chance, or have others made similar experiences?
Depends heavily on your ride:
For slower planes with NA engines I fully agree that FL50 is typically the sweet spot. For faster planes with Turbo engines in my experience the “as high as possible” is typically true, as long as the climb is less than 20% of the total flight time.
Btw: To really optimize this one also has to keep in mind that wind direction also changes with altitude – therefore depending on the specific situation going higher could even reduce the headwind component…
The optimisation would have been much easier if MET wind speeds at higher altitudes are corrected for density altitude to be treated like IAS instead of TAS 
then it’s only matter of finding +/- altitude delta around your sweet spot altitude with the highest “IAS tailwind” component, but you need to check that you have the required engine power & fuel flow to get high or enough ground clearance to get low !
On flying higher, yes even on nil winds it only make sense if the climb time is way less than x% (x being the typical IAS/TAS delta of the aircraft)
UdoR said:
Edit: I know that apps like Skydemon put out the optimal altitude, but it practically always tells me to go as high as possible…which seems not to be all the truth available out there.
Depends on what fuel figures you put in the profile, mine usually gives lower than I tend to go….
