I'm referencing my last post, so read down to get caught up.
Those of you who guessed there may have been some confusion with the Teterboro VOR and ILS frequencies, you win a cookie!
The inbound ILS course was set, but when the non-flying pilot read the chart and saw the TEB frequency in the 108 mhz range, he thought "108 is an ILS frequency", and tuned it and identified it. It's not quite that simple,so I'll paraphrase from Wiki:
VORs are assigned radio channels between 108.0 MHz and 117.95 MHz. Turns out the first 4 MHz is shared with the ILS band though. To leave channels for ILS, in the range 108.0 to 111.95 MHz, the 100 kHz digit (the first decimal place) is always even, so 108.00, 108.05, 108.20, and so on are VOR frequencies but 108.10, 108.15, 108.30, and so on, are reserved for ILS.
I didn't know that, but now I do. 108.4 would be a VOR because the .4 is even, where 108.9 would be an ILS because the .9 is not. I learn something new every day, and it's a good way to add a cross-check to the frequency being selected.
As we weren't expecting to see a glideslope indication (the G/S being out of service on this particular day), no warning flares went up right away.
Fortunately for us we were in good weather so we saw the airport a fair ways back, and got aligned with the runway, then discovered our mistake. Also fortunate for us, we were in a radar environment with ATC services, so if we had gone off-course in any major way (or even a minor way, this being among the busiest airspace on the planet), ATC would have let us know.
So how do we prevent this from happening again? Well, with SMS we ask ourselves a bunch of "whys".
Why were we off course? The wrong frequency was entered by the non-flying pilot on the approach.
Why was the wrong frequency entered? The non-flying pilot misread the chart and didn't see the correct frequency.
Why did he misread the chart? From his previous flight experience, he assumed a 108.** frequency would be an ILS (the desired approach), and didn't read further to the right of the chart where the ILS frequency was. It doesn't help that most people read from left to right, and the box on the left was the VOR, while the box on the right was the ILS. A contributing factor was that this was his first trip into the airport.
Why didn't the other pilot catch that in the approach briefing? Because our SOP's dictated that the non-flying pilot is solely responsible for entering the approach frequencies into our radios. A contributing factor was the Glideslope was out of service, so nobody was expecting to see it, the lack of which normally would have been an early indicator of something not-quite-right on the approach.
A-ha! The first part of that last paragraph seems to show a weakness on the part of our procedures - we had just one pilot in charge of confirming the information in the box, and while it had served us for a decade, this incident showed us that it could be improved. I immediately issued a memo stating that from now on, both pilots shall confirm the proper radio frequency is entered into the navigation radios prior to the start of the approach. We have added standard phraseology to our approach briefings "Radio tuned and identified to ***** (whatever the appropriate frequency is), Confirm?" The other pilot will physically point to the approach chart with the desired frequency and then to the nav radio, then both will listen to the proper approach ident, followed by "Confirmed". This will put both pilots more in the loop when it comes to entering this data into the box, which will add an extra layer of protection into our approach procedures.
Tomorrow we will talk about another aircraft that *appears* to have made a chillingly similar error, but with considerably more tragic results. It's pretty fascinating, and from our recent experience I totally get how it could occur. I'm talking about First Air flight 6560, which crashed in August of this year. More tomorrow.