avAltimeter User's Manual
The Apple iPhone 6 and later contains a barometric sensor which allows us to calculate the local pressure altitude and adjust the local pressure for variations in Sea Level Pressure just as one would with the Kollsman window setting in a real aircraft instrument. Even better, you can enter the outside air temperature into the app and it will calculate the current Density Altitude. With the Density Altitude we can make rough approximations of the True Airspeed relative to the Indicated Airspeed, and a very rough approximation of the change in take off distance given the density altitude, temperature and type of propeller (constant speed or fixed pitch).
The pressure sensor in the iPhone is quite sensitive and able to detect elevation changes of 2 - 3 feet, but it's accuracy is probably not quite as good as a TSO instrument. The Apple hardware automatically smoothes pressure readings and this introduces a delay of roughly 1.5 seconds or more in the readout.
The Main View of the App is a sensitive altimeter with three hands:
- Hundreds (long and skinny)
- Thousands (shorter and wider)
- Ten Thousands (very short with hatched counterweight)
At startup, avAltimeter is reading "pressure altitude" with the altimeter setting at 29.92 inches of mercury or 1013.23 mb. Once the current altimeter setting is obtained from a METAR or ATIS, avAltimeter is adjusted for corrected local pressure by tapping the adjustment knob in the lower left part of the gauge or the Kollsman Window. Set the desired value in the selector, then tap the adjustment knob again. Please note that the app limits the Kollsman setting to realistic values of atmospheric pressure! The allowable values are 870-1080 mb or 25.69-31.90 inches of mercury.
To change the Outside Air Temperature, tap the OAT box and enter the new temperature in Fahrenheit, then tap the DONE button.
The calculated density altitude is displayed to the right of the OAT.
Federal regulations require the use of supplemental oxygen when the ambient pressure is less than certain values. Above 12,500 ft, oxygen is required after 30 minutes above that altitude. Above 14,000 ft, oxygen is required immediately for flight crew. Exceeding 12,500 ft, avAltimeter will start a count-down timer with a progress bar on the screen and the Oxygen Warning indicator will flash. Exceeding 14,000 ft will immediately turn the Oxygen Warning solid red. An audible voice warning and vibration can also be enabled on the Setting page. After returning to a pressure altitude below 12,500 MSL, the warning progress bar will turn yellow for five minutes and then will reset the oxygen warnings. The five minute recovery time is not an FAA requirement, just a common sense suggestion.
True Airspeed (TAS) is estimated from the density altitude as function of Indicated Airspeed (IAS)
Estimated take-off distance (T/O) is a function of the take-off distance at sea level under standard conditions (59 °F) (ISA = International Standard Atmosphere). For example, a T/O of 1.2 ISA means that your take-off distance will be about 20% longer than your distance at sea level and 59°F. The Take Off distance calculation is from a rule of thumb created by Sparky Imelson, a well known Mountain Flying guru.
This page allow customization of the avAltimeter App. These settings are remembered between launches of the App.
- Kollsman Data Entry Controller
- This control provides a number of options which customize the "Altimeter Setting"."
- inHg - set Altimeter setting in imperial units, for example 29.92 inches of mercury
- mB - set Altimeter setting in metric units, for example 1013 millibars or hectopascals
- GPS - set Altimeter Setting automatically from the GPS derived altitude. Generally, the GPS altitude will be off up to a hundred feet or possibly more from the true elevation, so Be Prudent in using this feature.
- FieldElev - Enter the current, known Field elevation of your airport and the app will calculate, approximately, the correct Altimeter Setting, so that the Altimeter reads the correct altitude.
- METAR - Coming soon! The app will use wifi or cell data and GPS to automatically obtain the official Altimeter Setting value from the closest National Weather Service reporting station.
- Alt Display: Meters or Feet. Selects the units for the Altimeter and Density Altitude display
- Lighting: Day or Night. Dims screen for night operations
- Alarm Mode: Enables Audio and Vibrate Oxygen Alarms
- Propeller Type: Constant Speed vs. Fixed Pitch adjusts Take Off distance calculation
avAltimeter App V1.07
Copyright © 2015-2017 Aviametrix, LLC, All Rights Reserved
Calculations to determine altitude from raw pressure courtesy of NXP Semiconductors Inc.
Calculations to determine density altitude courtesy of Richard Shelquist, Shelquist Engineering.
Calculations for estimating Take-off distance courtesy Sparky Imelson (RIP) of Mountain Flying, LLC.
The iRate app rating module is courtesy of Nick Lockwood and Copyright 2011 Charcoal Design