Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

Help settle an argument about DA/MDA

Rotorheads A haven for helicopter professionals to discuss the things that affect them

Help settle an argument about DA/MDA

Old 9th Aug 2015, 22:05
  #41 (permalink)  
 
Join Date: Apr 2009
Location: Hotel Gypsy
Posts: 2,831
DB, you are incorrect. I can think of a number of scenarios where the ground on the approach is at a higher elevation than the threshold thus presenting a radalt indication of less than 200ft. The ICAO standard for MSD in the final approach segment involves a complex calculation which generally employs specialist modelling software, but the absolute minimum obstacle clearance on a precision approach within the final approach segment would be in the order of 130ft. ICAO DOC 8168 Vol II, Part II, Ch 1 refers.

PS. I concur with the rest.
Cows getting bigger is offline  
Old 9th Aug 2015, 22:25
  #42 (permalink)  
 
Join Date: Apr 2000
Location: EGDC
Posts: 8,193
As I understand the procedure construction - a non CDFA approach will have a step-fix to protect you from obstacles intruding into the approach plane so that it is safe to descend immediately to MDA/MDH unless a step fix is there.

Setting the radalt to anything other than ground proximity warnings (100' and 200' for precision and non-precision approach) adds complexity to the process (more calculations required) and is a further distraction if you are slightly tardy in your initiation of the GA - the last thing you want in a high workload situation is the radalt audio bleating.

I find it difficult to understand why the need to initiate GA above your DH/DA is now accepted as best practice for IR - it makes a mockery of the whole idea of Decision height/altitude as you won't fly to your minima - that minima which has been carefully constructed to permit flight to DA/H, then make a decision - then initiate the GA. If you start the GA at 50' above the minima you probably won't get to your actual minima so why have it in the first place.

It rather smacks of something that has been intellectualised rather than kept in its intended simple state.
crab@SAAvn.co.uk is offline  
Old 10th Aug 2015, 01:33
  #43 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 72
Posts: 4,268
I appreciate that the vast majority of responses here relate to the UK, but Boudreaux Bob has already given the Jepps decode which categorically rebuts the use of the rad alt, and my references are to my 'home ground' of Australia. As already mentioned, many ILS runways here have the surrounding land significantly below the threshold height thus rendering a radalt reading unreliable at DA/MDA.

This is still an International Forum!

Originally Posted by Pete O'Tewbe View Post
Whilst PEC may indeed apply, I'm not sure that this is what is being discussed here. If you are applying a PEC of, say, 50 ft, what was a DA of 400 ft now becomes 450 ft. In any event, PEC does not apply to NPAs with a published MDA, at least in the UK.
Pete O'Tewbe, the use of PEC relates to whether one is published for the aircrat in use: the +50ft is applicable in Oz for aircraft that do not have a published PEC. It is derived from the practicality that a published PEC will be of a low value, so 50ft is a reasonable impost on the lag of an baralt during a precision approach. If there is a PEC then you should apply that.

Have you ever checked whether your aircraft has an approved PEC, and the ramifications if it doesn't?
John Eacott is offline  
Old 10th Aug 2015, 03:36
  #44 (permalink)  
 
Join Date: Dec 2006
Location: UK and MALTA
Age: 57
Posts: 1,248
CGB the MSD you refer to alows for PEC aircraft position error, equipment tolerance and the GA manoeuvre.

John Eacott are you seriously suggesting that flying the GS at the very few international airports that have ILS in Oz you will be less than 200 feet MSD during the approach! I know its hard and trendy to accept greater risks down under but really!

You guys can hypothesise all you like. For me I will execute an immediate GA if my RADALT tells my I am at 170 feet MSD during an ILS where the state minima is 200 feet.

For all the usual heros out there that want to be as low as you can and think you can stay safe...good luck.


DB
DOUBLE BOGEY is offline  
Old 10th Aug 2015, 04:08
  #45 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 72
Posts: 4,268
Originally Posted by DOUBLE BOGEY View Post
CGB the MSD you refer to alows for PEC aircraft position error, equipment tolerance and the GA manoeuvre.

John Eacott are you seriously suggesting that flying the GS at the very few international airports that have ILS in Oz you will be less than 200 feet MSD during the approach! I know its hard and trendy to accept greater risks down under but really!

You guys can hypothesise all you like. For me I will execute an immediate GA if my RADALT tells my I am at 170 feet MSD during an ILS where the state minima is 200 feet.

For all the usual heros out there that want to be as low as you can and think you can stay safe...good luck.


DB
I'm not at all sure how you have devolved such a hypothetical from what I said. I have tried to point out (along with cows getting bigger) that using the rad alt will not give an accurate height above the airfield datum nor the threshold when used (as seems to be the inference here) as a warning at the 200ft point on the GS. The land under the aircraft at that time may be lower (as in the the case with many ILS airports here, not just international) or higher, and therefore you will not get a 200ft radalt warning at that point.

I have not said that you will be below 200ft at the appropriate point of the approach. Methinks you do protest too much and not read what is written, including Boudreaux Bob's link to the Jepperson decode which firmly rebuts the use of the radalt. In the same way that you misread my explanation of the +50ft penalty when a PEC is not available
John Eacott is offline  
Old 10th Aug 2015, 07:19
  #46 (permalink)  
 
Join Date: Dec 2006
Location: UK and MALTA
Age: 57
Posts: 1,248
John, reap what you sow! I have never advocated using DH to achieve the minima on a QNH approach! It is you who needs to read posts more carefully.

Ther is no ILS in the world with a state minima of 200 feet height above the threshold that would put you below that height, anywhere on the approach provided you are on the glideslope. That is made clear in pans-ops.

However, you are permitted some glideslope error at DA, accepting this at DA you may well be below 200 feet height above the threshold.

In modern helicopters fitted with a reasonable AP, coupled to the G/S such errors are eradicated.

All I advocate is that if the helicopter is fitted with a RADALT it is poor airmanship not to use it as the safety device it is clearly intended to be. To effect this it needs to be set below the listed DH minima for the approach, giving a margin to reduce the possibilities of the aural warning before the DA minima is reached. This is the design concept behind the AIRBUS bug system in the EC225/EC175 and it works well in practice.

The ILS systems in Oz do not have significant obstacles in the final approach. If they did the state minima for that approach would be greater than 200 feet DH CAT 1.
DOUBLE BOGEY is offline  
Old 10th Aug 2015, 07:46
  #47 (permalink)  
 
Join Date: Apr 2009
Location: Hotel Gypsy
Posts: 2,831
DB, you are incorrect. PANS OPS make no such assertion regarding 200ft MSD prior to DH if on GP. Take a look at:

Northolt RW25 - TDZ 124, 0.55nm (3.5 deg GP) 131ft
Gloucestershire RW27 - TDZ 87ft, 0.55nm (3.5 Deg GP) 102ft
There's many more where your radalt at DH will be less than baro alt (QFE). There are a few more exotic ones where the scenario appears at greater range.

Use of Radalt in IAP design is also mentioned in 8168. Personally, I set radalt to 50ft on an IAP but that's because I'm normally calibrating the ILS and 50ft is a good point to start roll-out guidance on a Cat III.

We digress, I see where you are coming from and would support the use of the radalt bug to increase SA.

Last edited by Cows getting bigger; 10th Aug 2015 at 08:22.
Cows getting bigger is offline  
Old 10th Aug 2015, 08:16
  #48 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 72
Posts: 4,268
Originally Posted by DOUBLE BOGEY View Post
John, reap what you sow! I have never advocated using DH to achieve the minima on a QNH approach! It is you who needs to read posts more carefully.

Ther is no ILS in the world with a state minima of 200 feet height above the threshold that would put you below that height, anywhere on the approach provided you are on the glideslope. That is made clear in pans-ops.

However, you are permitted some glideslope error at DA, accepting this at DA you may well be below 200 feet height above the threshold.

In modern helicopters fitted with a reasonable AP, coupled to the G/S such errors are eradicated.

All I advocate is that if the helicopter is fitted with a RADALT it is poor airmanship not to use it as the safety device it is clearly intended to be. To effect this it needs to be set below the listed DH minima for the approach, giving a margin to reduce the possibilities of the aural warning before the DA minima is reached. This is the design concept behind the AIRBUS bug system in the EC225/EC175 and it works well in practice.

The ILS systems in Oz do not have significant obstacles in the final approach. If they did the state minima for that approach would be greater than 200 feet DH CAT 1.
Round and round we go. Obviously you are seeing something in my posts which I am not.

I will try again. Using (as I believe that you advocated) the radalt bug set at 200ft to indicate that you are at or below the DA of 200ft can be erroneous. Many ILS approaches that I am familiar with come in over low ground (eg valleys) such that the glideslope height AGL is significantly greater than 200ft, even though the aircraft is on glideslope and is at 200ft above the airfield reference (be it threshold height or airfield datum).

Therefore setting 200ft (as you advocate) when the actual height AGL may be more like 250 - 300ft at that point serves no purpose. Since most ILS (and all NDA) precision approaches are designed to cross the threshold at 50ft, the ideal use for a radalt setting would be more beneficial if using a known reference point rather than a spurious and inaccurate one. My posts have been trying to make the point that the surrounding land is lower than the threshold, I see you are more concerned with land being higher: you are quite correct that this would create a higher DH, but that is not the point I am trying to raise.

If all the approaches that you are familiar with have surrounding surfaces level with the airfield, then this concept may be a difficult one to understand. But making silly remarks about Australian procedures is not a good way of discussing the issue, and I note that you have gone quiet on the PEC concept?
John Eacott is offline  
Old 10th Aug 2015, 08:19
  #49 (permalink)  
 
Join Date: May 2001
Location: UK
Posts: 145
Pete O'Tewbe, the use of PEC relates to whether one is published for the aircrat in use: the +50ft is applicable in Oz for aircraft that do not have a published PEC. It is derived from the practicality that a published PEC will be of a low value, so 50ft is a reasonable impost on the lag of an baralt during a precision approach. If there is a PEC then you should apply that.
I have not disagreed with anything you have said with regard to PEC. Except that with regard to the DA -0ft +50 ft discussed earlier, it is nothing to do with PEC.

Have you ever checked whether your aircraft has an approved PEC, and the ramifications if it doesn't?
Yes, and it is 21ft.

Thank you for your concern.
Pete O'Tewbe is offline  
Old 10th Aug 2015, 08:29
  #50 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 72
Posts: 4,268
P O'T,

Is there a similar requirement in Europe/UK for application of PEC to the DA? It is everything to do with DA here since it must be applied (or 50ft in lieu) to an ILS, which I believed to be the topic under discussion.
John Eacott is offline  
Old 10th Aug 2015, 08:48
  #51 (permalink)  
 
Join Date: May 2001
Location: UK
Posts: 145
From AIP AERODROME AD 1.1 AERODROME/HELIPORT AVAILABILITY:

4.6 Altimeter Error

4.6.1 When calculating Decision Height (DH), account must be taken of the errors of indicated height which occur when the aircraft is in the approach configuration. Details of the Pressure Error Correction (PEC) should be available from the aircraft Flight Manual or handbook. In the absence of this information a PEC of +50 ft has been found to be suitable for a wide range of light aircraft and should be used. This addition of 50 ft need only be applied to DH. The required RVR should be calculated prior to applying the
PEC.

4.6.2 The use of a radio altimeter is only applicable to approved Category 2 and Category 3 operations. For an aircraft flying a Category 1 or non-precision IAP, DH/Minimum Descent Height (MDH) is indicated on the pressure altimeter. At DH/MDH any readings from a radio altimeter may be unreliable because of the large area of terrain providing return signals to the instrument.
since it must be applied (or 50ft in lieu) to an ILS, which I believed to be the topic under discussion.
Again, I have not disagreed with you.

Presumably, you are now going to introduce temperature corrections into the debate?
Pete O'Tewbe is offline  
Old 10th Aug 2015, 08:52
  #52 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 72
Posts: 4,268
Originally Posted by Pete O'Tewbe View Post
Presumably, you are now going to introduce temperature corrections into the debate?
Not at all: you just did
John Eacott is offline  
Old 10th Aug 2015, 09:00
  #53 (permalink)  
 
Join Date: May 2001
Location: UK
Posts: 145
Touché!....
Pete O'Tewbe is offline  
Old 10th Aug 2015, 13:31
  #54 (permalink)  
 
Join Date: Dec 2006
Location: UK and MALTA
Age: 57
Posts: 1,248
Hi John. I have not advocated setting the RADALT to the DH. It is set usually 30 feet below that value with the minima set on the DA bug. We have the ability to set both bugs but the aural warning is only activated on reaching the DH bug. Hence the safety backstop.
DOUBLE BOGEY is offline  
Old 10th Aug 2015, 13:38
  #55 (permalink)  
 
Join Date: Dec 2006
Location: UK and MALTA
Age: 57
Posts: 1,248
CGB - the values you quote refer to height at the TDZ (touchdown zone), I have not checked but I suspect this is the TCH (Threshold crossing height). This is much further on than the Decision Altitude, which, acknowledging errors that may be present, is a minimum 200 feet height. If obstacles or higher ground are present this DH is modified accordingly.

I am no expert on approach design but the TDZ does not makes sense. Can you elaborate what this means?
DOUBLE BOGEY is offline  
Old 10th Aug 2015, 15:48
  #56 (permalink)  
 
Join Date: Mar 2005
Location: Holly Beach, Louisiana
Posts: 916
As this is an International Forum, the concept of Temperature Correction for very cold temperatures is not as esoteric as some might think.

Come up to Deadhorse in Alaska some Winter or any other location at either end of the World covered in White for Months on end.

If you think RadAlt readings matter during an ILS approach, just consider having a Sling Load hanging under the aircraft while doing the approach.

How does that figure into your concepts being put forth here?
Boudreaux Bob is offline  
Old 10th Aug 2015, 15:55
  #57 (permalink)  
 
Join Date: Apr 2007
Location: Center of the Universe
Posts: 644
From this link: Flying - US going back to TDZE on approach charts

"For international harmonization, US approach charts have been migrating to removing the TDZE altitude from approach plates and moving towards THRE as new approaches were designed and older ones were periodically updated. To date, over 4000 approaches have undergone this change, this is about 20% of the total.

The TDZE (TouchDown Zone Elevation) is defined as the highest elevation in the first 3000 feet of the runway starting at the threshold. The THRE is defined as the Threshold Elevation. Anytime the THRE is the highest point in the TDZE, they are the same. However, when the highest point is further down the runway, TDZE can be higher than THRE. If the runway meets standards, this can be as much as a 20 foot difference. In some rare cases where the runway is provided a variance, it can be more.

The THRE is not considered an operational altitude as one does not land on the threshold, but inside the TDZE. In US regulations, THRE is not mentioned, but TDZE is. In particular, on an approach (not Cat II or III) where the approach light system being in sight is used as the sole means of permitting descent below the DA/MDA, US regulations specify that they may only be used for the continued descent to an altitude 100 feet above the TDZE without meeting other criteria. With only THRE shown on the approach chart, this does not provide the data necessary to comply with the regulation."
EN48 is offline  
Old 10th Aug 2015, 19:39
  #58 (permalink)  
 
Join Date: Apr 2009
Location: Hotel Gypsy
Posts: 2,831
DB, I'll try to keep this simple (not that I'm accusing you of being simple).

AS EN48 says, the TDZ is the first 3000ft of an instrument runway and the notional touchdown point is found within this area. An ILS GP is set up such that an aircraft is guided down to a point in space that we would commonly understand as coincident with the TCH (in other words, a point about 50ft above the runway threshold). Beyond this point, the ILS GP signal is nonsense (it's actually pretty useless below about 120ft) but the notional GP is extrapolated to a point on the runway which is about one quarter distance into the TDZ (let's say 800ft). The ILS DME is then normally adjusted to give a zero reading at this point (try it next time you fly an approach - check the DME reading over the threshold and you will note is is normally a smidgen over 0.1nm)

The system minima is designed around the highest centreline elevation of the runway within the TDZ but the elevation of this point may not be coincident with the DME zeroed notional touchdown point. Indeed, in the most extreme cases, the highest TDZ elevation may be 2000ft further down the runway than the notional touchdown point; you could be short changed!!

What does this all mean?

It means that an ILS GP 'origin' is assumed to be a point some 800ft within the TDZ but the system minima is based upon the TDZE which will often be different to the elevation of the GP origin - the runway threshold plays no part. However, if we keep things simple and assume that TDZ, threshold and GP 'origin' are all the same elevation, we will find that a 3 degree GP crosses the threshold at about 50ft and will be 200ft at 0.65nm from the GP origin (about 0.52nm from the runway threshold). The two examples I gave, albeit 3.5 deg GP, demonstrate a higher ground elevation as an aircraft passes system minima and your radalt will, at this point, read less than 200ft (there are also buildings at these locations further reducing the radalt indication). I've personally flown one example (not in Europe) where the radalt was less than height above TDZE by nearly 100ft. London City also gives some interesting indications.

In simple terms, DAs/DHs aren't normally increased due to obstacles that occur on final approach prior to system minima as any such obstacle would probably demand an increased GP angle. Increased DAs/DHs are far more common due to problems in the missed approach segment which for a precision approach commences at system minima, i.e. the 200ft/0.65nm point.

I think I've rambled on; sorry.
Cows getting bigger is offline  
Old 10th Aug 2015, 21:46
  #59 (permalink)  
Thread Starter
 
Join Date: Aug 2001
Location: Cornwall
Age: 72
Posts: 1,305
CGB

An excellent contribution and one that will earn me a bottle of whisky from my erroneous colleague. He's also a great friend so will have to manage the situation carefully.
Geoffersincornwall is offline  
Old 11th Aug 2015, 02:17
  #60 (permalink)  
 
Join Date: Apr 2007
Location: Center of the Universe
Posts: 644
I have a colleague who believes that the number in brackets after DA on a Jeppesen approach plate is the ACTUAL rad alt reading at DA/MDA. I say it is the height above the runway reference (i.e. QFE equivalent).

I say you cannot use Rad Height as a guide to DA on a Cat 1 ILS. He says you can.

Who is right?
You are right. The number in parentheses on a Jepp chart is HAT (Height Above Touchdown Zone Elevation) See elsewhere in this thread for Touchdown Zone Elevation (TDZE). Radar altimeter is not a permitted means (and not an appropriate means) of determining DA on a CAT I ILS. (A CAT I ILS could potentially be designed to use radar altitude, but they are not, in part, because this would require a radar altimeter.) And on a non precision approach you may be bumping along at the MDA for some distance before reaching the MAP; the radar alt may be all over the place. On Cat II & III approaches radar altimeter is used and "RA" is shown in the minimums section of the chart. This is according to U.S. TERPS. Lots going on here; a link if you want to dig in: http://www.faa.gov/documentLibrary/m...s_1-26_rev.pdf Some differences may apply in other jurisdictions.

I found it useful to study TERPS carefully early in my instrument flying. Recommend that all instrument pilots at least give it (or the equivalent where you fly) some study, however, far more detail than you will need to keep in mind. What I found most useful is an understanding of the obstacle clearance provided (usually not much!) in critical phases of flight.

Last edited by EN48; 11th Aug 2015 at 02:37.
EN48 is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.