PRMG question

Are there any problems with PRMG and the auto land feature of SAU? I've been having a heck of a time just trying to do a PRMG approach of anykind. I have the correct channel, but when I set the mode to PRMG the blinker goes white. Any suggestions?

For PRMG to activate you will have to meet first glide slope parameters. john made a great review on that.

[ame]

I assume your are not reporting a bug..Cheers

Just tested some approaches at Krymsk from both directions. PRMG plus both SAU modes work OK, actually more correct than on very outdated John's vid above. SAU auto-land gives very smooth approach with gear up and flaps up. Much more hair-rising approach in landing config though - I guess I haven't figured correct speed for engaging system yet :D.

I just got my MiG-21 and played around with it for hours today... from what I understand, the PRMG transmitter on the ground has a rather narrow beam that it projects, meaning you need to be at the exact right spot to get the signal. My advice would be to intercept the final approach course (with RSBN) from a large enough distance (20km or more) and fly toward the runway. This way you will intercept the PRMG beam from below as you fly level. When you do, just enter the required descent to land.

[EDIT] Naturally the need for distance increases the higher up you are. Otherwise the beam is below you and you won't get a signal.

The PRMG system is a lot harder to use than it should be so don't feel too bad.

The -21 system has localizer limits of 23km +-3° both full scale and flag off. The L-39 can pick up the signal at 93km +-3° full deflection but it doesn't flag off the signal until 18° off axis. It uses this full range beyond displayed scale to command guide you to a 30 degree intercept.

The glideslope in the L-39 is good out to 79km, -21 is less than its localizer. The GS signal is by math ~2.65 degrees and ~3.66 on the -21.

It's still workable provided you intercept the beam in a good spot. Assuming you can stumble into it at 20km the GS beam would be >1000m. Coming in on a 30-45 degree angle at 600m QFE the localizer will come alive and quickly center. The GS should come alive from above and stay level until it crosses.

When both needles are inside 3 dots from center, engage the command mode and fly to establish the KPP command bars centered. When that's stable go to full automatic. The throttle is the only real control here. Provided you accept and compensate for the narrow and short range of the system it's quite good down to 50m. I definitely recommend getting the command bars aligned and stable on the "command" mode before engaging the automatic.

I guess transitioning from the RBSN Nav mode to Descent mode then to the PRMG landing mode has been my biggest problem. Especially when I need it the most...in IFR conditions. It seems like the information my instruments are giving me doesn't make sense. I'll keep investigating though.

Hmm... I should also check how the Descent-mode works... The tutorials only touch on Nav and Landing modes. Perhaps transitioning from Descent mode (which, if you believe the tutorials' brief note, is meant for cloud penetration... whatever that exactly means) to Landing mode is the issue.

Also, keep in mind, that on some airbases (e.g. MinVody, Nalchik), the PRMG signal is way off the runway center and If You follow it too long, it will run You into the ground :D. Use it just for a ballpark initial approach, but switch to visual as soon as You can.

I have succeeded in landing at Mazdok in less than a kilometer visibility with PRMG. To do so you really have to start at about 30km out lined up with the runway and about 1,800 m alt. and about 400-500 kph. In full landing configuration and in RSBN mode! When you get to 24km turn the mode switch to PRMG mode and hunt for the signal when you get both localizer and glide slope turn on SAU auto mode. You should hear the outer and inner markers as confirmation when you pass them. Only use Descent mode for the actual decent to 1,800 meters then go back to RSBN mode till 24 km out. Good luck!

I guess transitioning from the RBSN Nav mode to Descent mode then to the PRMG landing mode has been my biggest problem. Especially when I need it the most...in IFR conditions. It seems like the information my instruments are giving me doesn't make sense. I'll keep investigating though.

Indeed the 3 modes are a bit of a juggling act as each one is missing some desired information provided by the other ones.

Letdown mode is supposed to display horizontal guidance in addition to vertical guidance. I don't know if it's missing because it is actually constructed by the operator of the RSBN ground equipment (and therefore difficult to model) or is pilot-selected and overlooked. In any case the letdown mode indicates 10km altitude >120km and 600m <20km and various altitudes in between. You can think of it as a bowl with a flat bottom. You're also supposed to set the GROUND-AIR switch to air to ensure it uses the proper barometric reference (set in an avionics bay before flight).

Anyway, it should be no problem to do coarse lateral navigation with just the bearing pointer. It's probably best to do a box pattern and during thefinal "4" turn switch to landing mode. Since the localizer beam should be ~20 degrees wide (not in -21) we should get the localizer flag on the KPP to go black straight away. Continue the turn until the bearing pointer is 10 degrees from landing course and/or there is less than 30 degrees turn left to go and engage the command steering mode and confirm the K/T flags go away on the NPP.

The command bars should direct you for a level (not in -21, ignore the directed climb) turn to align. If you can't get the mode to stay on (green light burning) it's because -21 is requiring both localizer and glideslope to be valid (it shouldn't).

Fly the command bars to cross inside the middle bubble on the NPP and trim out the pressure. Then engage the automatic mode and monitor the T/K flags and command pointers during approach. Disconnect before landing. You can engage the auto mode directly but blend your stick inputs to help it onto course releasing the stick when it has centered the command pointers steadily.

I have the trk file for anyone to try and do a PRMG approach into Mozdok under the severe weather I've been trying to do it in. Good Luck! You'll need it...

Sorry. Here should be the file.MiG-21Bis Training.miz

I have to say the ILS is far superior to PRMG. ILS is much easier to pickup on final. Trying to pick up the PRMG in bad weather is like trying to shoot straight down a soda straw!

ILS and PRMG are nearly equally easy to use.

The spec on the localizer is "received at no less than 45 kilometers." And also usable in a ±15° wide region, the station being located ~1km beyond the runway in such a way that the first dot scale deflection corresponds to the lateral limits of the approach threshold.

The difficulty is we do not have a to-spec PRMG nor AFCS. If the localizer was truly available 15° from center in a nominal RSBN box approach then at say 20km downrange DME (really 23-24 km as the system is beyond the approach threshold 3-4 km) the localizer would be acquired 6.3 km laterally. For comparison a 500 km/h 30° bank turn has a radius of 3 km. You could switch to landing mode and engage the command AFCS green button all before executing the #4 turn. In fact the manual advises you not to engage the command button too early as its ??° banked turn command will cause you to cut the corner too much and fully align too close. Alternately disregard the bank pointer position until bearing is 10-12° from landing course.

The glideslope signal is not required for SAU command or automatic green button-light engagement. The SAU will command or fly a constant altitude (captured at moment of mode engagement) until the GS is valid and not above. The manual also says during transition from level to descent the GS position indicator may dip dots below when in automatic mode.

And throughout all this the SAU autopilot stabilization mode is available for the box pattern even if command steering mode is engaged. It's only incompatible with other automatic modes (approach automatic, danger altitude recover, leveling).

Despite all this a well executed small RSBN box approach into a PRGM command or auto landing mode will reliably allow landings down to 100m overcast ceilings. If one remembers that the signal zones are unnaturally small in range and width and prepares suitably (QFE, danger alt max, landing course set) it's really no difficulty.