Final segment descent gradients

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pstan

Pre-takeoff checklist
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Stan
Hi forum members, I'm hoping your cumulative wisdom can help me out here. I'm looking for two rnav gps approaches in the US with straight in landing minimums, one for each of the following specific situations:

Situation 1. The approach has no step down fix between the FAF and the runway. However the descent gradient, from the FAF crossing altitude, to the runway, is 3.1 degrees or more. I understand that the fms generated slope takes you (usually) to 50 feet above the threshhold.

Situation 2. Same as situation 1, however there is the presence of a step down fix requiring the gradient from the FAF to be increased (eg, the FAF crossing altitude could be increased) to 3.1 degrees or more, in order to comply with the step down fix altitude.

A few of us pilots have been having a discussion on exactly how our fms's handle such situations, afm says pilots are required to ensure altitudes at step down fixes are complied with. These two examples would sure help.

thanks to the group. I know there's tons of knowledge out there!!

Stan
 
I'll be interested to hear what the experts say too. I'm only a spam can flyer so the FMS issues are beyond me, but we all have to understand when the step-down altitudes are mandatory and when they aren't. I could be wrong, but I was under the impression that any crossing altitudes at step-down fixes inside the FAF applied only to the LNAV-only version of the approach. I would have assumed that if flying the LPV and cleared for the approach, I wouldn't need to worry about them.

For example, if flying the RNAV(GPS) 24 approach into KIKW (and cleared for the approach of course), I would assume that if WAAS was unavailable that I could descend to 1260 past BIRSE, but no lower until past EHAFY. But if the WAAS vertical guidance was available, I would just cruise down the glideslope and not worry about whether the altimeter read 1260 on crossing EHAFY.

Correct or no?
 
Situation 1: KSUN RNAV (GPS) W RWY 31 [3.11*]
Situation 2: KMTJ RNAV (GPS) RWY 35 [3.43*]
KBFI RNAV (GPS) Y RWY 13R [3.40*]

EDIT: Add situation 1
 
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Yep those apps are what I was looking for.

USN IE. the manual does say that it clears step down fixes, however now I can put those apps in and see the data myself

thanks Teller

Azure I think you are correct. any LPV or lnav/vnav app will clear any step down fix altitude past the FAF. With no vertical, eg Lnav only, then you would have to do it yourself
 
pstan said:
Yep those apps are what I was looking for.

USN IE. the manual does say that it clears step down fixes, however now I can put those apps in and see the data myself

thanks Teller

Azure I think you are correct. any LPV or lnav/vnav app will clear any step down fix altitude past the FAF. With no vertical, eg Lnav only, then you would have to do it yourself
Click to expand...

What kind of equipment do you have, Baro-VNAV or WAAS GPS? The Baro-VNAV system should clear the step downs based on the baro altimeter indication. The Baro-VNAV system should clear the step-downs because the glidepath will be affected by temperature to the same extent that the baro altimeter is affected, in other words the glidepath moves with the temperature. The WAAS GPS will nominally clear the step down fixes, but the glidepath is fixed and independent of temperature while the baro altimeter is affected by temperature and particularly on a hot day, you can bust a step down altitude.
 
John, I'm not sure whether I completely understood your answer. Are you saying that if you're cleared for the approach, following a LPV glideslope and there are step-down altitudes inside the PFAF, that the altitudes are still mandatory? I'm aware that on a warmer than standard day the barometric altitude may be above the GS, but I thought that the cases where pilots were busted for that all involved stepdown fixes outside the PFAF and separation issues due to underlying airspace. I thought the only reason for stepdown altitudes inside the PFAF was obstacle clearance.
 
azure said:
John, I'm not sure whether I completely understood your answer. Are you saying that if you're cleared for the approach, following a LPV glideslope and there are step-down altitudes inside the PFAF, that the altitudes are still mandatory? I'm aware that on a warmer than standard day the barometric altitude may be above the GS, but I thought that the cases where pilots were busted for that all involved stepdown fixes outside the PFAF and separation issues due to underlying airspace. I thought the only reason for stepdown altitudes inside the PFAF was obstacle clearance.
Click to expand...

I don't think there are any LPV or LNAV/VNAV approaches which have stepdowns inside the FAF for the approach when flying the GS, can you provide an example?

I think John was referring to the "advisory glideslope" available on some LNAV approaches (Garmin calls these LNAV+V). Your thoughts expressed above match my understanding of the potential for altitude busts on a LPV, ILS, or LNAV/VNAV approach, i.e. not an issue inside the FAF (or GS intercept as depicted on the chart) but possible if following the vertical guidance outside the FAF.
 
azure said:
John, I'm not sure whether I completely understood your answer. Are you saying that if you're cleared for the approach, following a LPV glideslope and there are step-down altitudes inside the PFAF, that the altitudes are still mandatory? I'm aware that on a warmer than standard day the barometric altitude may be above the GS, but I thought that the cases where pilots were busted for that all involved stepdown fixes outside the PFAF and separation issues due to underlying airspace. I thought the only reason for stepdown altitudes inside the PFAF was obstacle clearance.
Click to expand...

I agree that on an approach with vertical guidance, LPV or LNAV/VNAV, a step down fix after the FAF doesn't apply to the approach and would only apply to the LNAV procedure. I was primarily pointing out a characteristic difference between a Baro VNAV system and a WAAS GPS system in how the glidepath and altimeter interrelate to one another if you remain on the glidepath. It only has application when using the advisory glidepath function on a LNAV only procedure, but you can observe the difference when you are flying the LPV or LNAV/VNAV, even though it is of no consequence.

The primary difference is that with a Baro VNAV system that doesn't use temperature compensation, the glidepath will move with temperature and always remain in sync with the baro altimeter as they both will err to the same degree. This means that the statement "Using a Baro VNAV system and remaining on the glidepath will clear all step down minimum altitudes inside the FAF" is true. However, the statement "Using a WAAS IFR GPS and remaining on the glidepath will clear all stepdown minimums inside the FAF" is false as temperature will affect the altimeter indication, but not the WAAS glidepath. It is purely a technical point and would only have application on an approach that only had LNAV minimums.
 
Teller posted the rnav rwy 35 approach at MTJ Montrose CO as an example of situation 2, where the FAF crossing altitude would have to be raised in order to meet the step down altitude. However when I load the approach into our UNS 1E I see that the slope from the FAF crossing altitude of 8600 to the step down of 7100 feet is 3.43 degrees, and is the same as the slope from the step down at 7100 feet to the threshhold.

So know I'm not sure that this is an example that meets situation 2. Or has the procedure specialist already raised the FAF crossing altitude in order to ensure the step down altitude isn't busted?

So... For the situation where descent angle is greater that 3 degrees, does anyone know of an approach where descent from the FAF crossing altitude to the threshhold will violate a step down fix?

John, I'm using an "advisory" slope to lnav minimums. The UNS manual nor the AFM use the term "baro vnav", however I'm sure that's what it does. You say "The Baro-VNAV system should clear the step downs based on the baro altimeter indication." Is this true for all cases?

Thanks, Stan
 
Stan,

It is my understanding that the Vertical Descent Angle (VDA) that is published on the NPA charts along with the TCH will clear a step down fix inside the FAF, if it exists. The data is generated by the procedure specialist when they are developing the approach and I guess that mistakes can be made, but would expect this to be rare. I can think of at least one example where the step down is adjusted higher when used with the alternate baro setting location, and in that case, the glidepath will not take this into account.

From the TERPS, paragraph 252 Descent angle gradient:

Non RNAV approaches:

If the gradient from TCH to SDF (Step Down Fix) is greater than the gradient from TCH to FAF, continue the greater gradient to the FAF and adjust the FAF altitude accordingly.​
** ** **

RNAV approaches:

When an SDF is used, the SDF altitude should be at or below the published VGSI glide slope angle (lowest angle for multi-angle systems).

** ** **
​
[FONT=TimesNewRoman,Bold][FONT=TimesNewRoman,Bold]
Final With Stepdown Fix.​
​
​
​
​
The maximum descent angle is calculated using the difference between the FAF/stepdown altitude and the stepdown/TCH
altitude as appropriate. Descent gradient and angle computations apply to each stepdown segment. Height loss in the last segment flown is from the stepdown fix minimum altitude to the TCH​

[/FONT][/FONT]
 
Thanks John that is all very interesting, especially for the case of a simple rnav approach with LNAV straight in minimums, and the missed app point as the runway threshhold.

Your terps quotes seem to say that once you are at the FAF at the FAF crossing altitude, you can now descend on a constant slope to the TCH height at the runway, and not ever have to worry about any step down altitudes.

Do I understand this correctly?

If so, then why the term "advisory glideslope", and those that say pilots still need to take into account stepdown fixes/altitudes while on an "advisory" glideslope?

Stan
 
pstan said:
Thanks John that is all very interesting, especially for the case of a simple rnav approach with LNAV straight in minimums, and the missed app point as the runway threshhold.

Your terps quotes seem to say that once you are at the FAF at the FAF crossing altitude, you can now descend on a constant slope to the TCH height at the runway, and not ever have to worry about any step down altitudes.

Do I understand this correctly?

If so, then why the term "advisory glideslope", and those that say pilots still need to take into account stepdown fixes/altitudes while on an "advisory" glideslope?

Stan
Click to expand...

The TERPS may seem to say what you said, but they don't. There are two problems with your statement. On a NPA approach, it is true that the
Vertical Descent Angle that is calculated will nominally clear a step down fix most of the time. Why did I qualify my statement with "nominally" and with "most of the time"? I used nominally, because the VDA is fixed in space and is unaffected by temperature. That is not true for the altimeter that must be used to determine the altitude at the step down fix, it varies with temperature. Minimum altitudes are based solely on the barometric altimeter and not on the "advisory glidepath", so on hot days, if you remain on the advisory glidepath, you will indicate below the minimum when you reach the step down fix and this is not permitted. The most of the time qualifier is based on the rare example I offered in my prior post, where the VDA and step down altitude is based on using the local altimeter setting, and the step down altitude is moved to a higher altitude if a remote altimeter setting is used.

A more important point, is the conclusion that the VDA will clear obstacles after reaching the MDA and below, while continuing down to the TCH. That is absolutely false. The VDA will connect the FAF to the TCH, but there is no assurance that it won't plow though a mountain once you are below the MDA. The advisory glidepath is just that and it is only intended to be used for a stabilized descent between the FAF and the MDA. You are free to follow it or not. If you do follow it, you are still responsible for complying with any step downs and the MDA. Use of the advisory glidepath below the MDA can end your day very badly.
 
John, in my post that you responded to I made reference to a "simple rnav approach", in order that the effects of non standard temperature and remote altimeter setting would not be addressed. And in all my posts inquired specifically about "step down altitudes" and not potential obstacles to hit on the VDA but below the MDA. It seems to me then that a constant descent gradient from the FAF crossing altitude to the LNAV minimums, as provided by many gps/fms units (with the caveat you provided) will clear step down fixes.

I thank you for all your time and work addressing these issues. Excellent and clear explantions of the nuances of instrument approach techniques.
 
pstan said:
It seems to me then that a constant descent gradient from the FAF crossing altitude to the LNAV minimums, as provided by many gps/fms units (with the caveat you provided) will clear step down fixes.
Click to expand...

This is not true on hot days for a WAAS GPS system, for the reasons I previously stated. It is true for a Baro-VNAV system that does not use temperature compensation. Regardless, if you remain on the advisory glidepath and descend below a step down minimum altitude (which can occur if you follow the glidepath on a hot day), you are technically in violation of 91.177. In other words, don't depend on a WAAS/GPS system to clear a step down fix by following the advisory glidepath.
 
John Collins said:
This is not true on hot days for a WAAS GPS system, for the reasons I previously stated. It is true for a Baro-VNAV system that does not use temperature compensation. Regardless, if you remain on the advisory glidepath and descend below a step down minimum altitude (which can occur if you follow the glidepath on a hot day), you are technically in violation of 91.177. In other words, don't depend on a WAAS/GPS system to clear a step down fix by following the advisory glidepath.
Click to expand...

Seem like the FAA could remove that concern simply by ruling that it's permissible to go below the stepdown mins if necessary to remain on a published WAAS glideslope. There's no CFIT danger in this as the rocks and the WAAS altitudes are unaffected by the air temp. The only situation where a problem might exist is when the min altitude is there to clear traffic below the GS, and IME that's extremely rare if it exists at all.
 
gismo said:
Seem like the FAA could remove that concern simply by ruling that it's permissible to go below the stepdown mins if necessary to remain on a published WAAS glideslope. There's no CFIT danger in this as the rocks and the WAAS altitudes are unaffected by the air temp. The only situation where a problem might exist is when the min altitude is there to clear traffic below the GS, and IME that's extremely rare if it exists at all.
Click to expand...

All true, but I doubt that the FAA is going to come up with a rule change or ruling that only applies to a step down inside the FAF for an aircraft using the advisory glidepath that is not even a published part of the approach. The approach procedure as charted is regulatory by incorporation with FAR part 97 and one would have to rule that these regulations could be ignored. The advisory glidepath is not part of the charted approach procedure, it is an optional feature provided by the GPS manufacturer. If the approach has official vertical guidance (APV such as LPV or LNAV/VNAV), then any step down inside the FAF does not apply.

With regards to a glideslope outside the FAF, advisory or not, following the glidepath without regard for the minimum altitudes can create a conflict with traffic separation and is more likely to result in a pilot deviation.
 
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