We present new far-infrared line observations of the planetary nebulae (PNs) NGC 7027, NGC 7009, and NGC 6210 obtained with the Kuiper Airborne Observatory (KAO). The bulk of our data are for NGC 7027 and NGC 7009, including [Ne V] 24 μm, [O IV] 26 μm, [O III] (52, 88) μm, and [N III] 57 μm. Our data for [O III] (52, 88) and [N III] 57 in NGC 7027 represent the first measurements of these lines in this source. The large [O III] 52/88 μm flux ratio implies an electron density (cm-3) of log Ne[O III] = 4.19, the largest Ne ever inferred from these lines. We derive N++/O++ = 0.394 ± 0.062 for NGC 7027 and 0.179 ± 0.043 for NGC 6210. We are able to infer the O+3/O++ ionic ratio from our data. As gauged by this ionic ratio, NGC 7027 is substantially higher ionization than is NGC 7009, consistent with our observation that the former produces copious [Ne V] emission while the latter does not. These data help characterize the stellar ionizing radiation field. From our [O IV] and [O III] fluxes, we are able to show that O++ is by far the dominant oxygen ion in NGC 7009. As a result, the O/H abundance inferred using these data tends to corroborate the value found from UV/optical, collisionally excited lines. We determined accurate rest wavelengths for the [Ne V] 2s22p23P1 → 2s22p23P0 (λrest = 24.316 ± 0.008 μm) and [O IV] 2s22p 2P03/2→2s22p 2P01/2 (λrest = 25.887 ± 0.007 μm) transitions from observations of one or both of the bright PNs NGC 7027 and NGC 7009. Our [O IV] value, to the best of our knowledge, is the most accurate direct determination of this λrest prior to the Infrared Space Observatory (ISO). These new KAO data will be beneficial for comparison with the ISO observations of these PNs.
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