Localizer Approaches with autopilot

[sideburn 0]

Hey guys,

 

I have a quick question that I havent been able to find the answer to in a search. When I do a localizer only (not ILS) approach with autopilot on and LOC selected, What is the best way to maintain the correct descend rate. Currently I am using the altitude setting on the MCP along with V/S FPA and then monitoring it on the MFD until I get a couple hundred feet or so from the runway and then I turn off the autopilot and throttle down and land.

 

I am wondering if there is a better way, for example can I keep the localizer enabled so I can stay on a more precise track to the runway and manually controll my throttle and pitch. It seems like if you want to have the localizer enabled then you also have to have the altitude / VS controlled by the autopilot as well.

 

In the real world do pilots turn off everything and go manual when they are near the aiport? And if so how soon is it common to do this?

 

Thanks,

 

Tavis

 

 

[brucek 38]

Hi Tavis,

 

Most RL pilots that I know, including myself, far prefer to do a manual approach to landing than one on autopilot- after all, the approach and landing phase are the most challenging and therefore the most fun- so why let the auto-pilot have all the fun? Having said that, there are times when it may be prudent or even necessary to use the autopilot, including in IMC to assist the pilot, and in a low visibility (some CAT III landings) it is necessary to use the AP.

 

For a non-precision approach (such as a LOC approach), descent rates are usually expressed in feet/ nautical mile. (I assume you have access to charts, if not you will never know what your minimum altitudes are). The reason that charts express descent rates as feet/nm is that the chart designer doesn't know your ground speed.

 

But as you no doubt know, the avionics in the airplane usually refer to descent rates as "feet/minute". Presuming you know your ground speed, to convert feet/nm to feet/min, the formula is : Feet/minute = ground speed/60 X "feet/nm". So, for a descent rate on the chart of 300 feet/nm, the feet/minute for a C172 at 90 knots ground speed is 90/60 X 300= 450 feet/minute. That's what you would set your V/S in your autopilot to (-450), if for some reason you prefer autopilot approaches.

 

Bruce.

[jcomm 5,930]

Most of the approaches I have made in the jumpseat on A320/21/19s were handflown right from the IAF. Usually AT was disconnected even before the GS was intercepted, and from there on and until landing, all was manual!

 

The approach charts have descent rates published based on approach speed :

 

[brucek 38]

Yes- I should add to jcomm's excellent reply, that there is usually a table on the chart (Jepp charts) , or accessible within the book of charts , that gives you the descent rate in ft/minute. I'm a math major so I always prefer to hard way

 

Bruce.

[sideburn 0]

Thanks, 

 

Yes I have been looking at the charts and trying to calculate the descend rate. 

I also wanted to confirm that there is no way to have the autopilot ONLY handle the LNAV portion with LOC on while having the V/S disengaged so I would only be manual on the altitude. I guess that isnt possible though...

[brucek 38]

Thanks, 

 

.......I also wanted to confirm that there is no way to have the autopilot ONLY handle the LNAV portion with LOC on while having the V/S disengaged so I would only be manual on the altitude. I guess that isnt possible though...

I believe it depends on the type of autopilot, which in turn depends on the type of aircraft. What you are asking about is a piece of the training that instrument rated pilots go through (I remember it well!), and is usually done on a simplistic single or twin trainer aircraft so that you learn the instrument stuff without trying to also understand the complexities of advanced aircraft- I don't know if you are doing that or not.....

 

Bruce.

[jcomm 5,930]

As Bruce points out, there may be exceptions, but I don't know any AFS capable of setting only the LNAV mode active, while allowing for manual vertical control, but there are CWS ( Course wheel steering ) modes where the pilot can make his own inputs ( withing acceptable ranges) by pressing a button on the yoke, and the AP will take back control when the button is no longer pressed, and, above all, most pilots fly the approach using the Flight Director bar(s) as a guidance, either under low visibility precision or non-precision approaches manually flown or simply for trainning.

 

The Flight Director will give the pilot an indication of the lateral and vertical inputs required to maintain a given path, being it an approach or a route segment.

[sideburn 0]

I believe it depends on the type of autopilot, which in turn depends on the type of aircraft. What you are asking about is a piece of the training that instrument rated pilots go through (I remember it well!), and is usually done on a simplistic single or twin trainer aircraft so that you learn the instrument stuff without trying to also understand the complexities of advanced aircraft- I don't know if you are doing that or not.....

 

Bruce.

Thanks for the info everyone, I've just been hooked on flight simming for the last month or so.. But Ive been digging all over the net and reading everything I can to learn the ropes. So I started out on the cessna and learned about VOR navigation etc etc.. Now I have the Boeing 777 and have a pretty good understanding of a lot of it now but I there is still plenty more to learn.