Dead reckoning while flying is hard even today. If you are flying under IMC (instrument meteorological conditions, pilot slang for "bad weather/can't see shit"), you generally have to trust that your meters and instruments are working. Without GPS (or ground based triangulation methods), you generally have to guess where you are. It's very easy to accidentally end up in a small country like Switzerland.

If anyone's looking to build a startup in aviation, try building an alternative to pitot tubes. It's pretty much the last remaining vacuum instrument that hasn't been replaced by a solid state/MEMS alternative. And vacuum instruments are about as reliable as Windows Vista.