U.S. Bureau of Air Commerce report into the crash of United Airlines Flight 4 on 17 October 1935.
U.S. Bureau of Air Commerce (part of the U.S. Department of Commerce)
1935-10-31
National Transportation Library, U.S. Department of Transportation. <a rel="nofollow" class="external free" href="https://rosap.ntl.bts.gov/view/dot/32967">https://rosap.ntl.bts.gov/view/dot/32967</a>. DOI 10.21949/1500071.