ILS PEC and Climb to LSALT

[shimuken]

I have read text books and AIP but I can't find requirements on climbing to LSALT. Within what distance from departure aerodrome do you have to be above LSALT and when can you find it in the rule book? IFR pilots out there, please give me an answer.

Thanks.

Also how do you apply PEC for ILS DH? and what "unexplained discrepancy" means in relation to ILS ALT checks at outer marker? +/-60'? if so, more than +/-60' higher or lower, either way reqires missed approach or to continue with LLZ?

I couldn't find any of these info in my 10 years old Trev Thom text. AIP is current.

[Chimbu chuckles]

The info you want about terrain clearance is in the CAOs...section 20.

Given you are asking these questions I will assume you're a new IFR pilot flying very basic aeroplanes so from a practical point of view;

1/. You are responsible for terrain clearance up to the MSA/LSALT/MORA. including if you lose an engine. This means you will have studied appropriate charts/maps and know where the high bits are..hills, towers, aerials etc.

2/. You do what you have to do. The info you have available, apart from the charted spot hts, is various circling minima for various categories of aircraft out to 4.9 odd nm, then the 10nm and 25nm MSAs, LSALT on track and MORAs off track.

3/. Published Missed approach procedures for the runway you are using in combination with cat a,b or c circling minima can be a good way of getting from the runway to the MSA.

Just say, for arguments sake, you are departing on a pitch black night from a runway at 200' amsl, broken cloud at 500' and where there is nothing but flat terrain for 20nm in the direction you are going and then hills giving an MSA of 5000' and a track LSALT of 5500'. It would be perfectly legal, safe and reasonable for you to simply depart as you would in day VMC as long as you were above the 25nm MSA in plenty of time and then you must be at or above 5500' before exceeding 25nm. Or you might decide to takeoff, fly a circuit and depart overhead at say 2000' to ensure you make MSA/LSA in plenty of time...depends on aircraft performance, MTOW or just you and half fuel etc...in a Baron or Duchess or whatever I would depart overhead to make the engine failure after takeoff/return to land case as easy as possible...and build in buffer.

Pressure Error Correction is a correction for air flow annomalies around the fuselage which cause erroneous altimeter readings...or may cause.

If the manufacturer doesn't provide a specific correction then you add 50' to the minima as a catch all. Most modern jets, for instance, have a PEC of 0...I would be adding 50' if flying a Baron or similar.

Altimeter check hts on an ILSs are to check for;

1/. Correct QNH set on the subscale...i.e a gross error check.
2/. On the correct glidesope rather then a false one.

You must also understand that pressure sensitive altimeters are calibrated for ISA conditions...i.e. 15 degree C at MSL and a standard lapse rate above that...so the only time True Altitude = Indicated Alt is when it's ISA at the temperature datum (airfield you are doing the ILS at).

If it's hotter than ISA your TA will be higher than IA which is safe so no big deal. If it's colder than ISA your TA will be LOWER than IA which can be dangerous under conditions of extreme cold....ISA-15 or 20.

You cannot, in my opinion, carry out a meaningfull altimeter cross check without knowing what the ISA Devn is on the day.

A little rule of thumb formula for you.

TA=IA +/- 4' per degree of ISA devn x (Alt/1000)

In this formula Alt=Ht above the temperature datum which is always the airfield.

example; Arriving at YMML on a 35 degree C day. ILS check ht 1500'/5 DME. For simplicity we'll assume YMML is exactly at MSL....thus it's ISA + 20.

TA= 1500' - (4x20)x(1500/1000)
TA= 1500' - (80 x 1.5)
TA= 1500'-120'

So if you are exactly on the glideslope your indicated altitude will be 1380' but your True Altitide will be 1500'.

THIS is the figure you base your altimeter check on NOT the published figure on the approach plate. To this figure you apply any known error in the altimeter...perhaps you know it always under reads by 20' which would be quite acceptable for IFR operations...you did check it before departure right?

Just because you have the GS nailed means nothing...you could have captured a false GS and be at 500'/5nm and there is NO OTHER check other than an alt/dist at some point in the approach...you could fly into the ground/hit a building/tower or aerial 3nm before the runway with the glideslope indicating absolutely dead center....very rare but happens.

To give a little more understanding of the ISA devn relationship with varying HTs above the temp datum at the minima the ISA DEVN would be

TA=IA-4'x20x(200'/1000)
TA=IA-(80x.2)
TA=200-16'

Once again when your altimeter (indicated alt) says 200' your TA will be 216'

So at the CAT 1 minima it makes so little difference as to be irelevant.

I'll leave you to do the maths for the ISA- case.

Bottom line if the altimeter check is grossly in error check your altimeter subscale has the correct QNH set...maybe you forgot. If that is OK but there is a significant UNEXPLAINED error, particularly low...GO AROUND...you have either intercepted a false glideslope or the glideslope transmitter is making a direct attempt on your life:ok:

Editted for Zhaadum...thankyou, good catch. I got my columns of air, TA & IAs mixed up:ok:

[Zhaadum]

Chimbu,

A very informative post for our friend Shimuken.
However I see a slight flaw in the TA calculation example at YMML.

Assuming you were exactly on the correct glideslope at the OM check, wouldn't the Altimeter show 1380ft? (IA; 1500-120ft=1380, & TA=1500).
If Station Temp is ISA+ altimeter underreads compared to true altitude.

In your example, if the indicated Altitude was indeed 1500 and True Altitude 1620 you would now be way above glideslope on the ILS.

Cheers!


Z.:ok:

[shimuken]

I did my initial CIR long time ago and re-studying, could not find any of those info in my text or AIPs.
Thank you for very informative post.

[Chimbu chuckles]

Shimuken,

Sorry about that...Zhaadum's correction to my above post in the case of an ILS is correct. Should have known better than answer your post after midnight LT rather than this morning.:ugh:

Where I fecked up was confusing columns of air with passing over a fixed point on a G/S beam.

The way to think about an ILS is imagine you have a radalt and all this is happening over dead flat terrain. When you pass over the OM or reach the DME distance on the ILS if you are on the CORRECT GS your radalt will say 1500' (TA), your Altimeter will read 1380'. Thus your True Altitude is higher than your Indicated Altitude which is safe. Same at the minima...because you only reference Indicated Alt on a Cat 1 ILS you would have commenced the Missed Approach at 200' IA which would be 216' on your radalt..which again is safe. Please note that in the real world you could NOT use a Radalt at the OM/DME Ht check in this way...RADALTs can only be used on Cat 2, 3 and 3a approaches...in this case I was merely using radio altitude as a thought experiment to help understanding.

The reverse is the case in ISA -.

Where this gets dangerous is not the precision approach case...even at ISA - 20 it makes so little difference at a CAT 1 appraoch minima as to be negligable.

Because the IA/TA difference gets bigger the higher you are above the temp datum the real risk is high MSAs, LSAs and MORAs.

As an example I regularly fly over the Austrian Alps in winter enroute LHR and FRA. LOWW (Vienna, approximately 500' amsl) is the closest airport some 80 odd nm away. I can't remember the exact MORA on that route but it's about 14000'. Winter time temps at LOWW are regularly -5 to -8.

So ISA at LOWW would be 14 degree C.

At - 8 that would be ISA - 22.

If we experienced a rapid depressurisation over the alps would it be safe to descend to 14000' in IMC?

Nope!

From that little rule of thumb above; TA=IA- (4x22)x(13500/1000)

-88 x 13.5=-1188'

So at an indicated altitude of 14000' our TRUE ALT would be 12812'....we'd be 1200 odd feet below MORA.

If this sounds confusing given or at odds with the ILS example...opposite even...don't panic.

On the ILS you are calculating what your IA will be at a TA while in the MORA case you are calculating what your TA will be at an IA....make sense?

[Naughty S]

The important things to remember for ILS
Brief yourself
1 Alt/Dist when you should start the slippery dip
2 OUTER MARKER
3 DA most company ops mans +50'

Fly the plane

Get the A/C stable Heading LLZ, G/s ROD
Check alt dist profile frrom plate if you are stable
For HSI set Hdg bug for missed app and if no contact push everything forward and follow the bug:ok:

PEC for BE55/58 -20 and BE76 -10 C402 -10 from memory