Student consistently high on glideslope

7 Answers

1. 
   

   
   

   Ian on Nov 16, 2011

   Assuming you are having the student do things in the order you mentioned, I would suggest changing the order a bit. Have him set up the airplane at 90kts with approach flaps, and trimmed for level flight. I think in a Skyhawk that this is around 2100 rpm, but it’s been awhile, so play with it to check.
   Then when he intercepts the glideslope, simply reduce power to maintain the proper descent. I think that is 1900 rpm, but again, it’s been a long time.
   The point is that you want him to have the airplane on speed, with flaps set, and trimmed for 90kts before intercepting the glideslope. That way the only action required at intercept is a power reduction. In a retract, lowering the gear when one dot below slows the airplane and helps make it happen.

   0 Votes 0 Votes  0 Votes

2. 
   

   
   

   Nathan Parker on Nov 16, 2011

   “It seems he hasn’t taken my advice )which I’ve said multiple times) to set the power at a rough setting of around 1600 RPM, get on the glideslope, then trim the airplane so that it will hold the glideslope.”
    
   The student may have a scan problem, but it sounds like he doesn’t know how to adjust the yoke and throttle to correct glideslope deviations, which is understandable.
    
   When flying the glideslope, the absolute worst thing to do is to make attitude adjustments by referencing the glide slope needle.  That instrument is far too sluggish for precise aircraft control.
    
   The most important point to communicate to an instrument student is that flying the ILS is a constant rate descent manuever.  At 90 knots GS, you need a descent rate of something like 454 feet per minute to stay on the glideslope.  In order to intercept the GS from above, you need to increase that descent rate; to intercept the GS from below, you need to reduce that descent rate.  Flying the ILS is about increasing or decreasing the descent rate to intercept the proper glide path.  The VSI is a great tool for this.
    
   Now, the VSI has a slight lag too, but if you correct even slight deviations from the target descent rate, you can keep the GS needle centered. But the pilot can achieve an extra level of precision by using the instrument that has no lag and will keep the VSI nailed to a descent rate:  the attitude indicator.  With a constant power setting, a fixed attitude will result in a precise descent rate which will result in a precise glide path. 
    
   Therefore, glide path deviations should first be corrected by adjusting the pitch attitude, which will adjust the descent rate, which will then adjust the glide path.
    

   +2 Votes 2 Votes  0 Votes

3. 
   

   
   

   John D. Collins on Nov 16, 2011

   Power + Pitch + configuration equals performance. The typical glidepath is 3 degrees which works out to 318 feet per NM. At 120 Kts of groundspeed, it will take a descent rate of 640 feet per minute to remain on the glidepath.  For 90 Kts of groundspeed, it will require 477 feet per minute to remain on the glidepath. Each aircraft will have its own power, pitch, and configuration to maintain the desired groundspeed and rate of descent.  As an example, for my Bonanza, with the gear down, pitch set to approximately two degrees below the horizon, I will get about 120 Kts and 640 feet per minute with a power setting of 16 or so inches of manifold pressure. If the actual rate of descent is lower than the 640 feet/minute, I can expect to end up above the glidepath.  To correct back to the glidepath, I set the pitch a small increment down, say 3 degrees below the horizon, and allow the descent rate to exceed the 640 feet per minute so the aircraft will close in on the glidepath.  If the descent rate doesn’t increase above the rate needed to hold the glidepath, I must do something to correct this or there is no way I can re-intercept the glidepath. Once on the glidepath, I will adjust the pitch to hold the glidepath. In a similar manner, if I am too low, I will increase the pitch a small amount to reduce, but not eliminate, the descent rate, say to 400 feet per minute. Once the aircraft is stable and in trim, it will normally require very little control input to keep it on the glidepath.  The ILS approach charts have a timing table except if they are a ILS-DME to aid with figuring out the rate of descent verses the groundspeed. If the approach doesn’t provide the data, it is simple math to figure it out, a good rule of thumb is 5 times the groundspeed, so at 90 Kts x 5 = 450 ft/min.
    
   This is just an example, your airplane will be different and may normally do the approach at a different configuration, speed and rate of descent.  You can either provide the student the power, pitch, and configuration information or they can work it out themselves. Once you have the right target values, the student should be able to make appropriate corrections.

   0 Votes 0 Votes  0 Votes

4. 
   

   
   

   Wes Beard on Nov 16, 2011

   I agree with Nathan and what others have said.  I would like to add that it seems to me the student hasn’t realized the localizer / glideslope needles are not primary instruments.  You change the primary instruments based on what the needles are saying a very specific amount and never chase the needles.
    
   The heading indicator is primary for the localizer needle (bank) and the VSI is primary for the glideslope needle (pitch).  If one of the needles starts to move consciously decide to change the primary instrument.  In your case, a conscience effort to maintain 600 FT/MIN descent to reintercept the glideslope is far better than trying to control pitch with the attitude indicator; which is what I think they are doing.  The numbers may be different for your aircraft but I think you get the point.

   0 Votes 0 Votes  0 Votes

5. 
   

   
   

   Micah on Nov 17, 2011

   EastCoast, have you identified exactly what is happening with the student? This can be hard to determine, but your description makes me think of several scenarios:
    
   1. The student may not conceptually understand what is happening; I would spend time chair flying with the student and the chart until you are confident he / she can describe to you what you expect to see in the airplane.
   2. The student may simply have bad scanning technique / skills. In addition to what is mentioned above, I would go all-out on the partial panel to force the scan wider.
   3. Maybe the student isn’t the Type-A high-performance pilot and “is ok” with being 1-dot off… until uh-oh, it’s full scale. In this case, force a tighter standard and call go-around/missed approach when your student reaches 2 dots deviation. This may frustrate the student, but it should teach an appreciation for tight standards.
   4. Is the student afraid of “what could be down there” and turning fears into physical distance? That’s not something I’ve frequently observed, but it’s not uncommon to see pilots who create new dangers all for the sake of “safety.”
    
   Keep it up, and let us know what you find out.
   ~

   0 Votes 0 Votes  0 Votes

6. 
   

   
   

   Kris Kortokrax on Nov 18, 2011

   One other possibility might be that if the throttle position is being maintained, the power increases as you descend.  In the same way as you need to increase throttle to maintain a certain MAP in a climb, you need to decrease throttle to maintain MAP during the descent.  If the adjustments are not made, power increases during the descent which could contribute to trending high on the glideslope.
    
   I wholeheartedly agree with what the others have said about scan problems and the amount of deviation the student appears willing to accept before initiating a correction.

   +2 Votes 3 Votes  1 Votes

7. 
   

   
   

   David Eberhardt on Nov 23, 2011

   Kris makes a good point about slight decrease in power as descent progresses …
   If your basing the approach on 90 kts ground speed, a certain rate of descent will be required (as has been mentioned). You have to analyze the winds to see if you are really gonna have 90 kts G.S. or maybe it’s routinely higher (tailwinds at altitude???)
   In any event, it sounds like the student is consistently leaving the rpms too high therfore, getting high on the glidepath on a consistent basis.
   Power controls descent rate and pitch controls airspeed. I use the same method on visual approaches and instrument approaches in single engine acft like Cessna and Piper trainers.
   If I’m getting a little high on G.S. – power comes back 100 rpms and I ease the nose down 1/2 degree (and trim nose down a tad).
    

   +1 Votes 1 Votes  0 Votes