Sign in to follow this  
Guest Kingair315

Keeping the glideslope until the runway

Recommended Posts

I run into a problem of keeping the glideslope until the runway during an ILS approach. Everthing is going fine during the first part of the ILS descent. Then at about 500 ft AGL the glideslope goes down (and the aircraft descents with it), followed by the glideslope going way up (and the aircraft trying to follow this). I eventually have to cut AP and land manually, but most of the time not having a kiss touchdown.I am mostly flying the PDMG 737NG. The procedure I follow is this:- before the ILS approach I select the appropriate Vref and flaps in the FMC (since I am mostly quite loaded with passengers I tend to select flaps 40 and Vref of around 147 knots)- before the localizer intercept I have brought speed down to around 165 knots (with autothrottle), flaps 30 and gear down- I arm the spoilers and turn on the APR switch- then the localizer comes alive and the 737 turns to its final approach course (single channel)- when the glideslope start coming down and it is one dot above the target I select the final approach speed of around 155 knots (I like to be 5-10 knots above Vref for safety) and set flaps 40- the plane starts to follow the glideslope perfectly at around -800 feet/min- then at about 500 feet AGL the glideslope indication goes down and the plane descents with -1000/-1100 feet/min. Throttlepower is then at about 40-50% N1 to keep the planes speed at 155 knots.- the glideslope comes up again and now indicates that I am too low. It takes a moment for the plane to cut descent to -500 feet/min and the autothrottle to pick up power.- now I normally cut the AP at 200 ft AGL and try to land as smooth as possible (which does not always succeed as my virtual FSPassengers complain about a hard landing)What am I doing wrong?

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

If your airplane doesn't have auto land feature it's normal to disengage the AP at about 500 ft and hand fly the rest of the approach. I'd suggest continuing to do it by hand. If you are ready for it you should manage a smoother touch down than waiting for the AP to screw up first. Just disengage the AP at 500 ft and let it continue the way it's going and make very small adjustments to keep it on track. It's a lot easier than trying to save it once the AP looses the GS.Edit Personally if that happened to me I'd just go around and fly the whole approach by hand on the next try. That would do less damage to the airplane than a hard landing and less danger of totally loosing it. Don't try to save a bad approach. That gets you into trouble.

Share this post


Link to post
Share on other sites

In real life, not all airfields are autoland capable, generally larger aiports that are known to have bad weather needing such a system. In anycase, FS seems to recognize all fields as CAT III systems. You need to set both navs to the ILS frequency and activate both A and B CMD buttons on the MCP when on the localizer for autoland to work, otherwise it can't keep track of the glideslope when it starts getting narrower. Normally, you'd handfly the approach, or atleast under 500 feet for the normal CAT I ILS approach found at most fields.----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

Share this post


Link to post
Share on other sites

Thanks for the help. I'll try also to use the CMD B button next time. I was always wondering why there were 2 CMD buttons...

Share this post


Link to post
Share on other sites

I've noticed the same thing. It does seem like the glideslope is "bent" right before touch down. I say this because visually it seems the plane is staying at the same VS, IAS, and pitch, but suddenly the glide slope is diverging very quickly. ?????It doesn't seem like ground effect, but maybe???

Share this post


Link to post
Share on other sites

>I've noticed the same thing. It does seem like the>glideslope is "bent" right before touch down. I say this>because visually it seems the plane is staying at the same VS,>IAS, and pitch, but suddenly the glide slope is diverging very>quickly. ?????The weird thing is the very same thing happens with X-Plane, as well.Marco

Share this post


Link to post
Share on other sites

Checked this by using the slew function, the slope doesn't look as if it were "bent" somehow.It's more difficult to follow the ILS the closer the runway gets because the "cone" of the signal gets very narrow and thus the needles react more sensible.I seldom use the automatics for approach and landing, usually I have them switched off long, long before (the A/P takes the fun out of the game). It's a matter of personal likes I guess at which point runway lineup occurs (i.e. where the localizer is intercepted), gear is extended and what speed and flap selected (and what approach is choosen after all; at least that's my personal impression after many landings at smaller airports where I sometimes think there are no procedures publoshed because every airliner does it differently, and some even seem to be inclined to aerobatics a bit).I for my part slow down all the way during approach and thus lowering flaps incrementally (e.g. cross the localizer at 160, slow down gradually to 140-130 (some birds are so slippery (FeelThere 737PIC) that this can be a real challenge) until close enough to set almost full flaps and approach Vref; IMHO it's not good to fly with Vref all the way down, too much thrust needed and too little margin for corrections should something go wrong - speed is life, esp. in the DF727 or the 1-11 where I have the impression that engines react very slowly). I guess I reach Vref+10 when I'm about 1000 AGL or so. It's always risky to slow close to Vref 'cause speed tends to drop below it for me (happened to me this weekend at LOWI with the 737PIC in poor visibility, speedometer and VSI fluctuated wildly in the heavy turbulences).If I ever followed an ILS (what I seldom do the last months - VASI and PAPI are my closer friends ;-) ), I start to disregard it almost completely and fly the bird by hand and good judgement of speed and altitude (it's a thrill every time to get my 747 down at Funchal during bad weather taking the curved approach and be able to stop her in time).Andreas

Share this post


Link to post
Share on other sites

As has been said, on a standard ILS approach, it is completely normal and expected to cut the autopilot at minimums (usually about 200' AGL) and handfly to flare and touchdown. With a properly-done ILS approach, this should not take much more than gently pulling back on the yoke and cutting the throttle at about 50' AGL. You need to set the ILS frequency on both NAV radios and set both CMD switches on shortly before glideslope capture to enable autoland with automatic flare and touchdown.155 knots seems a little fast to me, with a 30-degree flap setting. Let alone 40 degrees... Are you landing with a very heavy load? Vref for me (as calculated by the FMC) is always in the mid 130's for a final approach speed about 137-143ish (Vref+5) with 30 degrees flaps (I only use 40 in certain cases). If I were to take a guess, this *might* be a reason for the glideslope becoming unstable at 500' AGL, because your airspeed is too high. If I'm doing a single-channel ILS approach, it's always rock-solid at least until I disconnect the autopilot about 200-300' AGL (after which it all becomes dependent on my flying skills :(). Either I'm consistently too slow, or you're too fast, and I'm almost always smooth as silk.... You might want to check that you're giving the FMC the correct information about the aircraft to ensure it will give you the correct Vref speed, because I think it's giving you a value that's a bit too fast...Hope this helps..

Share this post


Link to post
Share on other sites

For an autoland in the PMDG 737:Both NAV radios must be tuned to the ILS frequency and both CMD A and CMD B must be selected before 800 feet RAIf you select CMD A & B you should have "CMD" on the pfd above the artificial horizon. Leave the autothrottle armed with vref+5 dialled in.The rh flight mode annunciator should have "flare" armed under G/S and will change to "flare"Takes the fun out of it, but you'll autoland every time.Personally, I prefer the airbus voice system abusing me just before touchdown ("retard, retard")regards,Jeff

Share this post


Link to post
Share on other sites

This could be a side-effect of the way the glideslope may be modelled.Flight Simulator may model it simply by calculating if the aircraft is above or below a 3 deg angle (typically) relative to the location of the transmitter. If it does then the aircraft's position will appear to go below the glideslope just before touchdown. This is because the transmitter is offset from the runway centreline. In the limit, when the aircraft is on the runway alongside the transmitter then, because of the offset, it would appear below a 3 deg glideslope.I have noticed that at a particular airport the stock C172 will flare and effectively autoland. The glideslope needle begins to runaway commanding a pitchup just before touchdown.

Share this post


Link to post
Share on other sites

I'm with Andres on how to fly an ILS, or a non-precision approach. By manually flying it you also get to learn about an approach procedure. Both the localizer and the glideslope get very sensitive as you get closer to them, and since the localizer is 4 times as senesitive as a VOR radial anyway, buy the time you are under the 1,000' above touchdown elevation all you need is a small twitch of control movement to adjust a needle displacement. Remember that if one of the needles gets pegged and you still can't see the runway that this is a missed.Some airlines have procedures that require the aircraft to be stabilized in the approach at or before the 1,000' avove touchdown elevation point. This means that not only is the aircarft configured with flaps for touchdown, but that speed is also captured and maintained at that point.Bruce.

Share this post


Link to post
Share on other sites

I like to hand-fly my finals, even in big jets (the only big iron I fly a lot is the DC-8, but I hope to change that soon). I usually flip off the a/p about 10 nm out and do it the old-fashioned way. I almost always line up an ILS by hand-flying. Sometimes it's not easy depending on the winds.I'm not famiiar with the PDMG 737 but I can say for a DC-8-70 series, they are VERY speedy and you have to really stay ahead on your approach by throttling way down, or else you'll come in too hot. (It's not a good idea to use spoilers on final in a DC-8, either...)If you come in a little fast then it can be hard to maintain the ILS when you're in close. Unless I can't see the runway, I usually don't worry about being a bit above or below the ILS if I'm very close, because by that point being off by 5 feet can show a lot on the gauge. I might be all wet on this but I personally consider overly chasing the ILS to be a somewhat fruitless habit just like "chasing the VSI" is when trying to maintain level flight. Just so I get it down in one piece and relatively gently, that's all that counts in my FS. :)Rhett

Share this post


Link to post
Share on other sites

Generally, once you have sight of the runway and know that you will not lose sight of it again (in otherwords, if you know you're still going to be popping in and out of clouds, stay IFR oriented), you fly visually and the ILS isn't a factor, you then fly the VASI/PAPI/etc down.Here's how I generally fly the ILS approach in the FT 373PIC. I maneuver during vectors with flaps 1 and at 200 knots. When vectored for the localizer intercept, flaps 5, 10 knots above the flaps 5 speed listed on the speedcard or the tick on the speedtape. Once the glideslope is alive, gear comes down, flaps are set to 15 and I slow to 10 knots above the flaps 15 speed. At one dot hit on the glideslope, the last of the flaps come in (30 unless I forsee a need for 40), and slow to what ever speed I'm supposed to be at for the approach. If I'm doing an autoland with autothrottle on, it's Vref + 5 regardless of conditions. If I'm flying manually, I also kick the AT off, in which case my VA's SOP says to follow the glideslope at Vref + (half the sustained headwind component) + (the full gust factor), but not to be less than Vref + 5.Example 1:Headwind Component: 12Gust Factor: 10VREF 30: 128128 + (12/2) + 10 = 144Approach at 144 knots on the glideslopeExample 2:Headwind Component: 7Gust Factor: 0VREF 30: 128128 + (8/2) + 0 = 132Approach at 133 knots since the result of the calculation is less than 5 knots over VrefThis will vary from airline to airline. This is just how MidCon's SOP calls for it.----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

Share this post


Link to post
Share on other sites

>>If your airplane doesn't have auto land feature it's normal to >>disengage the AP at about 500 ft and hand fly the rest of the approach.200' is the basic IR license privilege. Airports may add on a bit more. But 500' is quite high for precision approach.

Share this post


Link to post
Share on other sites

If you fly into the same airfield consistently, it may be easier to practice. Landing at a different one each time, can make it more difficult as you wont develop the same visual "cues" about landing. So, generally am flying without the autopilot from 2000'For several years fly the Kingair and B1900 only, generally using the Vasi lights as a guide. Radar Contact vectors me close enough for visual landings. Instead of depending upon the glideslope it makes it easier for me to practice landing at any airfield depending on VASI, and I make all approaches at 120-140. This keeps me constantly aware of where I am or making corrections and thus can land at fields where there is no ILS or Glideslope just as easy as one where there is... So basically use the glideslope only when visibility is severely limited. Like the last few days at my home KFUL, there is a fire in the Anahiem Hills and the smoke it totally obscuring the field. As smoke does not show up in real westher, I just set my own weather to simulate the conditions.Bob

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this