Instrument Written Question

[gms5002]

Hey Guys - A question from a practice written test I hope you can shed some light on.

Refer to figure 227 (page 81: https://www.faa.gov/training_testing/testing/test_questions/media/ir_akts_addendum.pdf). Refer to the DEN ILS RWY 35R procedure. The FAF intercept altitude is: (A) 7080 feet MSL, (B) 7,977 feet MSL, (C) 8,000 feet MSL

It says the correct answer is 8,000 feet but I don't understand why. The explanation says:

8,000 feet MSL (lightning bolt symbol) is the glide slope intercept when using the ILS (precision) approach procedure.
Answer B is incorrect because 7,977 feet MSL is the glide slope intercept located at the FAF (final approach fix) when using the non-precision (localizer) approach.

This doesn't make sense to me...the FAF is in the same spot whether its and ILS or localizer only and it's clearly marked as 7,977 feet. It looks like the 8,000 foot intercept occurs before the FAF, then I would start down, crossing the FAF at 7,977. What am I missing?

 

[MauleSkinner]

The FAF for an ILS is the point of glideslope intercept at the intercept altitude (8000), whereas the FAF for the LOC-only procedure is the named fix...two different points over the ground.

 

[gms5002]

Ok, so for an ILS approach, the FAF is always the glideslope intercept and the maltese cross is not relevant? I guess I didn't realize that because I'm used to the lightning bolt pointing directly at the maltese cross. Thanks.

 

[RotorDude]

Ok, so for an ILS approach, the FAF is always the glideslope intercept and the maltese cross is not relevant? I guess I didn't realize that because I'm used to the lightning bolt pointing directly at the maltese cross. Thanks.

Actually it is relevant, as a cross check. You should normally ensure that as you cross the OM on the GS, your altimeter is roughly equal to the intercept value. So in the test example, you'd make sure that as you hear/see the OM signal, your altimeter reads around 7977'. If not, you might want to consider aborting the approach.

 

[poadeleted20]

Ok, so for an ILS approach, the FAF is always the glideslope intercept and the maltese cross is not relevant?

Insofar as the ILS approach is concerned, yes. However, it's still a darn good idea to hack the clock at the Maltese cross point so you know when to start the missed approach if the GS conks out halfway down final.

I guess I didn't realize that because I'm used to the lightning bolt pointing directly at the maltese cross.

When the GS intercepts the intermediate segment altitude at the FAF (as it I think it always does on RNAV LPV approaches and it usually does on regular ILS approaches), that's what you'll see. However, if the intermediate segment altitude intercepts the GS before the Maltese cross fix (as it does on the ILS 26 at MRB), you're considered to be on the final segment and you'll start down on the GS at the point that the GS centers. Note that as RotorDude mentioned above, on that MRB approach, you'd want to see the altimeter reading 2310 with the GS needle centered as you pass the HEVEN Maltese cross fix.

 

[gms5002]

Ok that makes sense, thanks.

 

[MauleSkinner]

Actually it is relevant, as a cross check. You should normally ensure that as you cross the OM on the GS, your altimeter is roughly equal to the intercept value. So in the test example, you'd make sure that as you hear/see the OM signal, your altimeter reads around 7977'. If not, you might want to consider aborting the approach.

A good example would be an LNAV/VNAV approach with a mis-set altimeter...baro-VNAV systems compare GPS position with baro altitude to compute a glideslope indication. The right combination can drive you into the ground well short of the runway while indicating "on glide path".

 

[c310flyr]

From the same approach plate, is that correct that besides the altitude being mandatory at 9000, the speed is mandatory at 210 knots?

 

[RotorDude]

From the same approach plate, is that correct that besides the altitude being mandatory at 9000, the speed is mandatory at 210 knots?

Correct, while crossing FIRPI.

 

[c310flyr]

So if you can't do 210.... ?

 

[BillTIZ]

The altitude profile only clears you from 9000 to 8000 outside the FAF. You cannot descend below 8000 without further guidance, either the GS or crossing a fix. For the ILS you'll get GS guidance and cross the FAF at 7977. For the LOC you stay at 8000 until the FAF then start down to 7080.

 

[Irish_Armada]

Ok that makes sense, thanks.

Good question. I was just recently struggling trying to figure out the same distinction.

 

[RotorDude]

So if you can't do 210.... ?

Then you can't use this approach. Plenty of others to choose from, I suppose. Or, in some unusual situation where all other approaches are unavailable, you could ask ATC to waive the speed requirement (since it's clearly a separation issue). But I stand to be corrected.

 

[azure]

Actually it is relevant, as a cross check. You should normally ensure that as you cross the OM on the GS, your altimeter is roughly equal to the intercept value. So in the test example, you'd make sure that as you hear/see the OM signal, your altimeter reads around 7977'. If not, you might want to consider aborting the approach.

Though depending on temperature, and how high above the altimeter setting station you are, being a couple hundred feet off might be completely normal. I almost aborted a practice ILS approach a few weeks ago when the GPS showed I had passed the LOC FAF but was still below GS at the indicated intercept altitude. After a few seconds I realized that the reason was that it was very cold, 25C or so below standard, and so the baro altimeter was reading higher than true altitude.

Just something to keep in mind.

 

[RotorDude]

Though depending on temperature, and how high above the altimeter setting station you are, being a couple hundred feet off might be completely normal. I almost aborted a practice ILS approach a few weeks ago when the GPS showed I had passed the LOC FAF but was still below GS at the indicated intercept altitude. After a few seconds I realized that the reason was that it was very cold, 25C or so below standard, and so the baro altimeter was reading higher than true altitude.

Just something to keep in mind.

Yes, good point. See for example here for relevant discussion. Personally, when crossing the FAF on a precision approach I would cross check both the WAAS GPS altitude and altimeter. If it's an extra cold day, I'd expect the altimeter to read high while the WAAS should always be spot on (within +/- 50 feet or better), if I am right on the glide slope at that point.

Edit: Another important point that I think is not emphasized enough in training is that when executing an ILS or LPV, you should not descend from the IF altitude until intercepting the glide slope. This way you are guaranteed crossing the FAF at the right (safe) altitude even on cold days.

 

[azure]

Yes, good point. See for example here for relevant discussion. Personally, when crossing the FAF on a precision approach I would cross check both the WAAS GPS altitude and altimeter. If it's an extra cold day, I'd expect the altimeter to read high while the WAAS should always be spot on (within +/- 50 feet or better), if I am right on the glide slope at that point.

And, just a nit, but I'd expect the GPS to read low rather than the altimeter to read high - since you still fly baro altitudes for separation reasons. Unless, of course, you're performing a cold weather correction and have informed ATC.

Which brings me to a totally off-topic question, but still might be of interest to people studying for the IR: are there any plans to include cold weather correction on the IR written? It wasn't even mentioned in any of my study materials a few years ago, now we have a NOTAM covering it, so it's definitely something every IR pilot needs to know. Certainly a lot more relevant today than being able to calculate ADF bearings.

 

[RotorDude]

And, just a nit, but I'd expect the GPS to read low rather than the altimeter to read high - since you still fly baro altitudes for separation reasons. Unless, of course, you're performing a cold weather correction and have informed ATC.

See my edited addition above that I think addresses this point.

 

[c310flyr]

Then you can't use this approach. Plenty of others to choose from, I suppose. Or, in some unusual situation where all other approaches are unavailable, you could ask ATC to waive the speed requirement (since it's clearly a separation issue). But I stand to be corrected.

Never been there but just curious. That's the only ILS shown.
They show 4 or 5 fbo's all of whom sell 100LL.

Not too many 100LL burners can do 210 kts on the approach.

Are the pistons really not able to do an ILS in general, without asking special permission ?

 

[RotorDude]

Never been there but just curious. That's the only ILS shown.
They show 3 fbo's all of whom sell 100LL.

Not too many 100LL burners can do 210 kts on the approach.

Are the pistons really not able to do an ILS in general, without asking special permission ?

Not sure about special exemptions, but there seem to be two RNAV approaches to that runway (with 200-1/2 mins). And with RNAV GPS Y 35R passing through the same FIRPI IF without the speed restriction, it seems odd that they would insist on that speed just for the ILS. Maybe someone knowledgeable can chime in.

 

[flyingron]

Never been there but just curious. That's the only ILS shown.

Vectors to final works. CASSE used to be an IAF, but it appears to have been removed in the past year or so (it may have been in error as it's placement there neither was marked with NoPT or any indication on a course reversal).

 

[azure]

See my edited addition above that I think addresses this point.

This really belongs in a different thread, but the whole point of the new requirements for cold weather correction is that on cold days you are NOT guaranteed adequate obstacle clearance even at the charted IF altitude (baro). It's all covered in this thread for those who are interested (and those of us who fly in colder climates SHOULD be interested).

 

[RotorDude]

This really belongs in a different thread, but the whole point of the new requirements for cold weather correction is that on cold days you are NOT guaranteed adequate obstacle clearance even at the charted IF altitude (baro). It's all covered in this thread for those who are interested (and those of us who fly in colder climates SHOULD be interested).

Yes, but I am not saying that you can ignore extra cold temperatures. My point was that if you do not start descending from the IF altitude until glide slope interception, you'd be guaranteed obstacle clearance from that point on, including when crossing the FAF. I agree that in those cases where the IF altitude is relatively low above the FAF, with a minimal obstacle clearance, you might want to apply the cold weather correction to the IF altitude if the temperature is significantly below normal. But the principle remains the same: don't descend from the (possibly corrected) IF altitude just because you may, wait until glide slope interception and once on it, you are home free.

 

[azure]

Oh, 100% in agreement about not descending from the IF altitude until intercepting the GS. And that once on it, you do have guaranteed obstacle clearance since the altitude on the GS is based on GPS location or a ground-based transmitter, not on baro altitude. I was just trying to clarify (mostly for IR students) that even the charted altitudes on the approach plate are not always guaranteed to have the required obstacle clearance.

 

[RotorDude]

Oh, 100% in agreement about not descending from the IF altitude until intercepting the GS. And that once on it, you do have guaranteed obstacle clearance since the altitude on the GS is based on GPS location or a ground-based transmitter, not on baro altitude. I was just trying to clarify (mostly for IR students) that even the charted altitudes on the approach plate are not always guaranteed to have the required obstacle clearance.

Agreed. As example, take the RNAV 27 into KSAN. Of course in this case the odds for temperatures significantly below normal are slim to none, but the principle remains the same (and I happen to know this approach well). VYDDA is the IF, and in theory you could start hopping down to each intermediate step, but the safe, correct and easy way is to simply stay at 4000' (the IF altitude) until you intercept the GS, and then just slide down safely to the runway, while ensuring you have the minimum altitudes at those intermediate fixes and verifying 2000' at REEBO (the FAF).

 

[James331]

Insofar as the ILS approach is concerned, yes. However, it's still a darn good idea to hack the clock at the Maltese cross point so you know when to start the missed approach if the GS conks out halfway down final.

I used to teach that, however in practice I don't.

For one I have the GPS programmed for the approach, which honestly is going to give me better timing than going off a chrono.

Second if anything starts acting stupid on the ILS I'm just going missed, into the hold, and checking my systems, looking at other approaches, etc.

 

[bobmrg]

Yes, good point. See for example here for relevant discussion. Personally, when crossing the FAF on a precision approach I would cross check both the WAAS GPS altitude and altimeter. If it's an extra cold day, I'd expect the altimeter to read high while the WAAS should always be spot on (within +/- 50 feet or better), if I am right on the glide slope at that point.

Edit: Another important point that I think is not emphasized enough in training is that when executing an ILS or LPV, you should not descend from the IF altitude until intercepting the glide slope. This way you are guaranteed crossing the FAF at the right (safe) altitude even on cold days.

Just a nit for you and azure: Check AIM 1-1-18(a)(4), where it tells pilots that they should not use GPS derived altitude to determine aircraft altitude.

Bob Gardner

 

[RotorDude]

Just a nit for you and azure: Check AIM 1-1-18(a)(4), where it tells pilots that they should not use GPS derived altitude to determine aircraft altitude.

Bob Gardner

I would never "determine aircraft altitude" using GPS, even if I know it to be far more accurate than the barometer, for two reasons. One, it's dangerous from the separation POV, and two, it's not legal. OTOH, nothing prevents me to assume the higher of the two, in case of disagreement between them (once cleared for the approach when on final approach or intermediate segment). This also holds true for the colored terrain depiction and obstacle alerts shown on the panel mounted GPS -- if it's showing red and alerting me, I am not flying into it, regardless of what the barometer says.

 

[azure]

I would never "determine aircraft altitude" using GPS, even if I know it to be far more accurate than the barometer, for two reasons. One, it's dangerous from the separation POV, and two, it's not legal.

100% agreed. Not sure what I wrote that led Bob Gardner to think that I would, either. Legally, you fly the baro altitude. If there's a safety of flight issue, of course 91.3 allows you to deviate and do whatever you need to. But I wouldn't normally just fly the GPS altitude, and if I needed to apply a cold weather correction, I would negotiate that with ATC. They might, for separation reasons, have to park me somewhere for traffic to pass.

Edit: after checking the AIM reference, that section seems to apply to TSO C-129 certified GPS receivers, which do not have WAAS. With WAAS in use, GPS altitudes are a lot more accurate than barometric, and in fact do integrity checking which that paragraph assumes GPS receivers do not do.

This doesn't change the fact that legally, you're still required to fly the barometric altitude.