[R] Importing fixed-width data

[James Rome]

I have a data set where the lines look like:
2011-05-13 00:00:00 EONAAL330 dfa13002516PSCNONA
2011-05-13 00:00:01 EONAAL223 laa13044510AS.NONM
Some lines are missing the field before and after the NON:
2011-05-13 00:00:05 EONBHS229 mia13001621NON

I read them into R using
    df = read.fwf(file, widths=c(19,-4,7,3,8,2,1,3,1),
       
col.names=c("DateTime","Flight","Dest","ArrTime","MsgType","Conf","Runway","Source"),
       
colClasses=c("POSIXct",NA,"factor","factor","character","factor","factor","factor"))

The documentation for read.fwf says that the data are read into a
dataframe. Yet, I get a list, and the conversions I specified do not
seem to have been obeyed:
> df[1:20,]
                         DateTime  Flight Dest  ArrTime MsgType Conf
Runway Source
1  2011-05-13 00:00:00 AAL330   dfa 13002516      PS    C    NON      A
2  2011-05-13 00:00:01 AAL223   laa 13044510      AS    .    NON      M
. . .
> sapply(df, mode)
   DateTime      Flight        Dest     ArrTime     MsgType        Conf
  "numeric"   "numeric"   "numeric"   "numeric" "character"   "numeric"
     Runway      Source
  "numeric"   "numeric"
> dfn = df[!is.na(df$Source),]
> mode(df)
[1] "list"

What am I doing wrong?

Thanks,
Jim Rome