Non-Precision Approaches

Description of your first forum.

Moderator: BKirkham

Non-Precision Approaches

Postby Dave Reed » Mon May 28, 2018 10:34 pm

A non-precision approach is any approach that does not have a glideslope associated with it. There are three basic types to fly: The localizer, the VOR and the NDB. The NDB is the oldest, least accurate and the one that is almost extinct today. Back in the day though, NDB approaches were the most common type of approach. Basically the NDB is just a tower putting out a signal, just like a radio station tower. In fact, when I was an instructor, we used to fly practice NDB approaches using radio station towers because it was not at the airport so you could do as many as you liked without the bother of dealing with approach control or local traffic.
The VOR approach uses a VOR or VORTAC (Variable Omni Range. TAC stands for TACAN, which is what the military calls a VOR). VOR approaches are easier to fly and more accurate than a NDB approach. You dial in the desired heading and follow the needle. I'm going to assume that everyone reading this knows how to track a VOR.
The localizer is twice as accurate as a VOR but only works in one direction. It's an accurate beam that is pointed in one direction and cannot move, so if the inbound course in 090, it will always be 090. A glideslope is simply a localizer turned on it's side, providing accurate vertical guidance. The localizer antenna is normally found at the far end of the runway, while the glideslope antenna is adjacent to the touchdown point.


Let's look at an approach. My example is the VOR-A approach to Chester CT, home of Chester Charter. I personally have flown this approach many, many times. It's called a VOR-A because it isn't aligned with any runway. If it was aligned with runway 17 it would be called the VOR-17 approach.
Look on the right side near the middle and find the MSA circle. This is the Minimum Sector Altitude. It is the minimum safe altitude within the arc depicted out to 25 miles from the depicted fix, in this case MAD (Madison VOR). If I was flying along and was going to do this approach without any help from ATC, I would descend to the altitude shown when I was withing 25 miles of MAD. Say I'm coming in from a direction of northwest, 300 degrees. My inbound course would be 120 degrees. My safe altitude would be 3100' msl.
Look on the profile view near the bottom. The approach starts at 2100'. If I were to cross MAD at 3100' and start the approach, I'd be starting 1000' too high. So I need to descend to 2100' in the holding pattern. This is simple. So long as you are within the holding pattern, you are safe. Cross MAD at 3100. The outbound course is 227 degrees. Turn to 227 and start down. Watch your clock. After reaching 227, start the timer for one minute. Don't do this at 300 knots! Best to be back to your approach speed when doing this. In my KingAir we fly the approach at 130 KIAS, so I would fly the holding pattern at 130 KIAS. While flying outbound, set your VOR CDI to the inbound course, 047. After one minute, turn right and intercept the 047 course inbound. One minute out, one minute in the turn, you should leveling at 2100' now. If you want to do it slow and carefully, drop approach flaps now.
When you get to the VOR, set the CDI quickly to 076, the final approach course. Turn to 076 and correct as necessary to track the 076 radial outbound. Looking at the profile view, descend to 2000'. So long as you are on course, you can follow the profile plan. Note the cross depicted at ALCAR. That is the Final Approach Fix, or FAF. As you come up on it (within 1 mile), drop the landing gear. At ALCAR (5 mi DME), start down to the minimum descent altitude, or MDA. At the bottom of the chart this is shown as 960'. The A B C D indicate category. At 130 KIAS you are flying in category B. Descend at 800-1000 feet per minute and level off at 960'. It's OK to level off a bit early (like 1000'), just don't go below 960' yet. Adjust power as needed to maintain 130 kts. When you reach MACOB at 9.4 DME, you either see the airport or you don't. If you do, cross over top of the airport and fly a standard traffic pattern. After turning left 90 degrees you'll be on a downwind for runway 17, or upwind for runway 35. Stay at 960' still! 960' is about 600' above ground level (AGL). If you start down from 960' when starting your turn to final, you will most likely be on the proper 3 degree glidepath. The VASI lights are good for this. When turning final drop full flaps and slow to landing speed.
If the approach were lined up with the runway, you would drop full flaps at the FAF. Descend at 800 fpm and when you get the runway in sight you will probably be right on the VASI. Think of it this way: Almost every approach, at the FAF, begins at 1500' above the runway. From here a 3 degree glidepath will bring you to the runway threshold. At 130 kts, you will want about 800 feet per minute to be on that glidepath. You can come down quicker, level off at MDA and wait for the runway to appear, but if you use 800 fpm then when the runway appears you will have very little changes to make. A VASI is great. Don't leave MDA until the VASI indicates you are on the correct glidepath (red over white).
A Localizer approach is the same thing, just more accurate horizontally. An LDA or SDA approach is a localizer approach that is not lined up with the runway. You see these in Alaska a lot because the terrain may not allow for a straight in approach.
Dave Reed
Site Admin
Posts: 292
Joined: Tue Nov 12, 2013 9:23 am

Re: Non-Precision Approaches

Postby PAA0691 » Sun Jun 03, 2018 9:28 am

As always, great info. I always get stressed with approaches :P
Posts: 99
Joined: Thu Aug 17, 2017 5:30 pm
Location: Buenos Aires, Argentina

Return to General Topics

Who is online

Users browsing this forum: No registered users and 0 guests