Would the weather wash out the evening’s session?
The rain was lashing down as I left to drive to Lyneham for this evenings IMC training sortie. Would the weather postpone today’s session? Fortunately not – the rain had stopped by the time I arrived, and the forecast was reasonably good.
It did mean that Ian, my instructor, could discuss what kind of weather limits would apply to an IMC rated pilot. The standard recommended limits of 600 feet cloudbase and 1800 metres runway visibility apply when landing. I suggested that this was too low to take off because weather could deteriorate further preventing you from landing again. Ian noted the larger problem with such a low cloudbase is the short time and choice available in the event of engine failure – trying to complete a forced landing from 600 feet would be extremely challenging. So he suggested a limit of 1500 feet cloudbase might be more appropriate. He also discussed what to do if the engine did fail when in cloud – descend quickly to build up a reasonable speed so that when you do emerge you have some energy which gives more choice to reach a field further away.
He also explained that many IMC pilots use the rating to allow them to climb through the cloud and break out into VFR “on top”. This considerably expands the potential for flights, especially to other parts of the country with better weather.
[As an aside, the recommended minimum descent limits for an IMC rated pilot are 500 feet for precision approaches (ILS or PAR) and 600 feet for non-precision approaches (NDB, SRA). These are the limits I plan to use and what most IMC course and instructors teach. But it seems that legally, there is no distinction between decision heights for Instrument Rated (IR) and IMC rated pilots – both can descend to the published limits of any instrument procedure. This discussion thread on the PPrune forum clarifies.]
Instrument Landing procedures available today
Obviously, the flying club’s activities have to take second place to the military operations at Lyneham. Availability of the instrument landings and other IMC procedures varies depending on the activities at the time. After a discussion with ATC it seemed that we would be able to do some instrument landings, so we booked out and were shortly heading off for departure. First we did an ILS approach, climbing up straight after take-off, we then turned back towards the airfield tracking the NDB. The published procedure involves tracking away from it at 88 degrees, so we had to fly towards it at +/-30 degrees from the opposite side (i.e. 268 +/- 30). I made the radio call and ATC cleared us for the procedure.
We descended down to 2600 feet while watching the ADF – the wind wasn’t nearly as strong or in such a crosswind on this leg, so that was less of a problem. With the DME tracking our distance from the NDB beacon, I called out the distance to go – making a 90 degree turn at 8 miles, then going straight ahead for 15 seconds, then turning again to make a 45 degree intercept with the ILS. It was strange to announce “localiser acquired” for the first time, but there we were ontrack. I won’t claim the tracking of the ILS was perfect, but I was correcting as we descended – the combination of ILS and DME really lets you know where you are and how far to go. As we reached 500 feet, I took off the foggles and we did a touch and go – not perfect, but good enough.
An SRA approach
We then went around and repeated the approach, but this time as an SRA (Surveillance Radar Approach). This is a “talkdown” approach where the radar controller tells you which way to steer (left a bit, right a bit etc.) and what your height should be (based on distance to the threshold). This is slightly different to a PAR (Precision Approach Radar) where the controller has a separate screen that shows your track on the glideslope and also advises you to descend more quickly or slowly. Typically PARs are only available at military airfields, but SRAs can be provided at many commercial airports.
Ian had explained that one difference with the SRA at a military airfield is that they may use the PAR equipment (without the glideslope readout), rather than the primary radar display which is used more commonly at commercial airfields.
Simply following the instructions, I think we tracked the approach fairly well and seemed to come out at the right place.
The NDB approach
The last one for the evening was the NDB approach – almost certainly the most difficult type – and for this one, Ian more or less left me to it. We tracked outbound the beacon and I used the DME primarily to determine when to start the procedure turn back towards the airfield. In the debrief, Ian pointed out that the NDB approach plate states “8 nm” rather than the “8d” in the ILS plate – the difference being that you would be expected to time the 8 miles sector from the beacon rather than use the DME alone. In practice, you are expected to make use of any (and all) available navaids, so it would be reasonable to time it but use the DME readout as confirmation. A final point on this is that the NDB beacon and DME are not co-located – we were using the ILS DME which is at the threshold of the runway, where the NDB is perhaps as much as a mile away.
Messed it up somewhat
I messed up the turn because I just put on the final runway heading and turned directly through to that. Light aircraft turn much more quickly than the larger planes these procedures are designed for, and this meant I was considerably to the left of track and had to correct with a substantial change to intercept the correct bearing. It also meant I couldn’t start the descent because I was outside the zone. I should have repeated what we’d done for the ILS, namely make a 90 degree turn, straight for 15 seconds, then turn again for a 45 degree intercept. By leaving me to my own devices, Ian reinforced the point and I will be much more aware of it next time. I also didn’t put the carb heat on for much of the approach – bad news if you are in warm cloud – there’s no problem in having this on throughout the descent, and you’d certainly not want to find yourself suffering from carb ice as you decide to go-around.
Transition to landing
We then continued the approach and I took the foggles off at about 600 feet, put full flaps in and landed in a calm wind. I’ve heard it said that the transition from IFR to VFR (ie when you take off the foggles or emerge from clouds) can be quite offputting – some instructors do the landings for IMC students. I’ve been lucky and not had this, which means I’m getting more used to the transition and the relatively short time left to finish the approach. I can see why having everything configured early is a good idea.
After a debrief, I’d say this was another successful IMC training sortie. By no means complete, but I think I’m getting the hang of it. My height keeping is still ropey and I will need further practice on these NDB holds and approaches, so Ian is promising me some challenging IMC tracking tasks on our next trip. We may also visit another airfield to get a different perspective too.
Flight Time today: 1:10
Total IMC training time to date: 11:40
(Minimum dual training time for the IMC course is 15 hours)