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I've had an iPhone for about 7 months now and it has definitely proven to be one of the most worthwhile devices I've ever purchased. In addition to the built-in features like email, calendar, contacts and web browsing, I love the ability to extend the device's usefulness with third-part applications. I wrote last year about switching to LogTen on my iPhone and on my Mac for logging all my flight times. I have to say this app is a must have, especially for Mac users.
I've found some other useful iPhone apps and here they are, in no particular order.
Filing Flight Plans
While DUAT provides a mobile device version of their web site, I don't find it much easier to use than the regular, non-mobile web site version. For a friendly, easy-to-read flight plan interface, nothing beats FltPlan. Access it using the web browser of your choice on your regular computer and create a user id at no charge. When you access FltPlan on your iPhone using Safari, login and you can get a quick and concise weather briefing (which meets the requirements of 14 CFR 91.103). You can file IFR flight plans very quickly and easily well as get access to radar summary and weather depiction charts, NEXRAD for an airport or a route, winds aloft, PIREPs and more. I don't have many suggestions for improving this site. The content has obviously been very well thought out, though the layout sometimes seems a bit crude.
Airport Directory Information
And another tip of the hat is in order to the folks at FltPlan for creating the free FltDeck Airport/FBO Info Guide app. I sure wish I'd had this on last summer's ferry flight since it provides lots of information on all airports in the continental US as well as Canada, Mexico, and the Caribbean. An important point to mention is that this is a stand-alone app: Once you install it, you don't need a network or 3G connection to access airport information and that means you can look up information while in flight with your phone in airplane mode.
Weight & Balance
Weight and balance spreadsheets have proven useful to me for years and I usually create one for each aircraft that I fly. I even created one years ago for the Caravan and it really simplified my life by letting me let the ramp crew know quickly and accurately how much cargo my plane could take that day. Wanting to weight & balance on the iPhone seemed like an obvious idea and Spreadsheet lets you do this easily. Create a weight and balance spreadsheet in Excel (I assume OpenOffice would work, too), then save a copy in XML format. You can use Spreadsheet's built-in file transfer feature to move the XML spreadsheet to your iPhone and you're in business baby! The original price has increased slightly since I bought it to $7.99, but it's worth it.
Surface Weather and TAFs
There are a lot of aviation weather iPhone apps out there and while ForeFlight is probably the best in breed, it's a little expensive for my current budget. I've tried a few of the cheaper apps, but for my money Aero Weather provides a lot of bang for little bucks - it's free! You create a list of favorite airports and you get a simple, concise, easy-to-read list of those airports. If you want to see the TAF for an airport, you can get that, too. And you have the option of raw or decoded formats. Remember this data does not meet all the requirements of a weather briefing for the purposes of 14 CFR 91.103, but it can give you a good overview of the weather situation in your area.
Another low-cost weather app is AirWx which only costs US$6.99. Like Aero Weather, AirWx lets you create a list of favorite airports, access METARS and TAFS, but it adds features like an E6B calculator and the ability to access weather charts, AF/D entries, and terminal procedures for many airports. It's not exactly what I'd call user-friendly and the E6B, while functional, is a bit cumbersome, but the price is reasonable.
While AirWx provides a usable E6B, PilotWizz does a bit better job. The price is either free or a low US$9.99 if you decide to buy the Pro version. It even has a holding pattern feature, though I think it's more useful as a teaching/learning tool than for actual use in flight.
I also use a few non-aviation apps (most of them free) that I can recommend, including:
Skype - internet telephony meets iPhone
Pandora - music
Shazam - music
iBart - for Bay-Area Rapid Transit train schedules
TouchType - for email and Twitter (once I'm ready to stream my consciousness)
Wikipanion - for reconciling bets and disagreements on facts, history ...
Tides - so I can walk my dogs near SF Bay without Taz disappearing into the mud flats!
Any aviation apps you use that I haven't mentioned? Please chime in, but be forewarned that spam and inappropriate comments will be sent to /dev/null.