Here's the text from the Apple web page cited above:
iPad USB power adapter certifications
Every iPad comes with an Apple USB power adapter that is certified to meet government safety standards in countries where iPad is available.
Many of these countries, including the United States, have adopted the Safety of Information Technology Equipment standard, IEC/UL 60950-1. Learn about the certifications for iPad USB power adapters.
Depending on the model, iPad comes with a 5W, 10W, or 12W power adapter. All three Apple USB power adapters are designed for use with power sources rated to provide 100V AC to 240V AC at 50Hz to 60Hz. Find more information about iPad USB power adapter compatibility.
The Type A (flat parallel-blade design) Apple 5W as well as the 10W and 12W USB power adapters (with Type A AC plug attached) also comply with the IEC/UL 60950-1 standard for use with power sources rated to provide 115V AC at 400Hz.
Published Date: Jun 21, 2017
Elsewhere Apple states that the iPad chargers can be used to charge an iPhone:
Using iPad power adapters with your iPhone, iPad, and iPod
Apple USB power adapters for iPad can also be used to charge iPhone, iPod, Apple Watch, AirPods, and other Apple products.
You can use Apple 12W and 10W USB power adapters to charge your iPad, iPhone, iPod, Apple Watch, and other Apple accessories, like AirPods and Siri Remote. Just connect your device to the power adapter with the Lightning to USB Cable, 30-pin to USB cable, or Apple Watch charger that came with your device.
https://support.apple.com/en-us/HT202105
But as everybody points out, it will probably work fine but might still be illegal unless you get a stack of approvals from the feds, the airline, the unions, the manufacturer etc.
Originally Posted by
Derfred
However, before you go and plug one in, you would also need to comply with whatever EFB approvals are in place by your operator to be approved to use that power outlet for your EFB.
And the original question was about using an iPhone, not iPad, power adapter on 400 Hz. I've seen it done with no problem in years past. But, it is probably not legal and not covered under Apple's warranty if the phone, adapter or cable bursts into flames.
