Density altitude and one's iPhone 8

Add 1000 ft for every 8C above standard temp. IME mostly gives you a little buffer on the safe side. I use that all the time and cross-check against the METAR (around here most stations report DA) and it works a treat. In any case, if a couple of 100ft either way really make a difference then perhaps you shouldn’t be flying that day.

Peter wrote:

Yes – see another post on this really good pilot forum

Oh, well!

Yes – see another post on this really good pilot forum

Airborne_Again wrote:

Is it really worth while to correct for humidity? I correct -27 feet/hPa deviation from standard pressure and 1000 feet/8°C deviation from standard temperature.

I just checked using this website. Apparently at sea level with standard pressure, 0% or 100% relative humidity will make a difference of 300 ft density altitude at 20°C and 540 ft density altitude at 30°C. That was actually much more than I expected. On the other hand even 540 ft corresponds to not more than 4-5°C temperature difference so it is unlikely that someone will crash because they didn’t take humidity into account.

aeroweather calculates the DA of any airport that has a METAR with said data.

QuoteI’d be interested in where the iPhone (8+ in my case) gets its barometric input, and whether a case is going to obstruct the port (if there is one).

The iPhone 6 and newer has a barometric sensor so that’s where it “gets it from”. I could not find any information on how accurate it is, but I would suggest getting an app that displays the raw pressure sensed and compare it to your airfields QFE over a period of weeks, to see if it works. My gut feeling is that it’s going to be extremely accurate, and temperature compensated as well. And I don’t think any sort of protective cover is going to have an impact on the reading, unless you put the phone in a water/airtight rigid case of some sort.

If you get an app which allows you to input the airfield QFE/QNH, then it should be able to display the actual height/altitude as well, and you can compare this to your altimeter for calibration. However, make sure you get an app which uses QFE/QNH and barometric input for altitude calculations: Most often the barometric input is used to correct GPS altitude, and GPS altitude is NOT what you want to calibrate the barometric sensor.

According to Apples API documentation, raw pressure (in kilopascals) is available from the sensor: https://developer.apple.com/documentation/coremotion/cmaltitudedata. They also output “relative altitude”, which is the altitude gain (or loss) since the initialisation of the sensor – which you’d normally do once you start the app. Again, no documentation on whether they follow ISA rules for this and how they deal with temperature compensation.

If you can’t find a suitable app, it’s relatively easy to whip up a one-page app that displays the absolute barometric pressure gained from the sensor, then an input box for QNH/QFE plus a derived altitude/height according to ISA rules (although automatic temperature compensation would be an issue), and then a block for GPS derived altitude. That would be ideal for seeing how accurate the sensor is over a series of flights. I could probably write such an app in a day or so, but I don’t have an iPhone 6 or later available so I would not be able to test it myself.

Alternatively, there are several “sensors” applications on the App Store that will display and record the raw output from all sensors on your iPhone. They’re not aviation specific so they don’t calculate altitude based on barometric pressure, QNH and temperature according to ISA rules, but they do show you the raw pressure from the barometric sensor. That may just be good enough for your purposes.

Telegram Aviator‘s Bot
https://t.me/AviatorsBot
Command ICAO DA eg EGLL DA

App AutoDens works with my iP 6S+.

Is it really worth while to correct for humidity? I correct -27 feet/hPa deviation from standard pressure and 1000 feet/8°C deviation from standard temperature.