Altimetry and Obstacle clearance,reminders..

[M.85]

Dear All,

Having made a few mistakes about Altimetry,I have ,last ight,hit the books...including JAR Ops 1.
You may find here a few refreshers for Altimetry problems and Obstacle Clearance,both theory and practical infos.
Here we go:

A few Defs:
Altimeter is a pressur instrument,it reads changes in Isobars(pressure)but not feet in between isobars.(we all know that)
QNH uses ISA to find MSL pressure(Altimeter reads height of airfield above MSL)
QFF uses actual temp to find MSL Pressure but gets less accurate with altitude
At MSL QFF>QNH in Colder air.

Altimeter errors:-instrument error(you get what you pay for!)
- barometric error(finger trouble or no reset when qnh has changed during flight)
-temp error(if temp other than ISA,altimeter in error,use high to low watch out below!)

____During Approach, To ensure adequate Obstacle clearance on apprapproach,add figure in body of table to calculated DH/MDH
(larger figure i found was at -65C from isa and MDA 1000ft above aerodrome:260ft of error.
At-65C of isa and DH 200ft above aerodrome:52ft error)

____During flight below transition level:

STD(1013)-given QNH gives the difference in pressure(MB) between PRESSURE alt and TRUE alt.
When temp are less than ISA,A/C will be lower than the altimeter reading,the error is approx 4ft/1000ft/1C of difference from isa.

BOAC..I have no clue how you found a 2000ft difference..it would be possible,apart from not updating QNH,if in ISA conditions:2000 divided by 4=500 then x 1000ft=500000ft above MSL(WHat do you fly??)

_____During flight above transition alt(FL):

To determine the minimum safe FL,you need to know the elevation of the highest ground/obstacle along your track,the minimum terrain clearance and the QNH.
Ie:Altimeter set @1013
Altimeter reading 8500ft
Local QNH 999
-->1013-999=14x27ft(ISA)=378ft.(if not ISA use 96xtemp K(273K=0c)divided by 1013)
A/C true alt is 8500-378=8122ft.

OBSTACLE CLEARANCE:
Def:OCH for Precision approaches lowest altitude(OCA) or height above the elevation of the relevant threshold(OCH)at which a missed app must be initiated to ensure COMPLIANCE with appropriate clearance CRITERIA.If A/C has DH(by ops)less than 200ft,operator must prove to Authorities A/C can climb a inimum of 2.5%gradient.

-----Take/off

The minimum obstacle clearance equals 0 at DER and thereafter increase by 0.8% of the horizontal distance in the direction of the flight,assuming max divergence of 15°.,In the turn initiation area a minimum obstacle clearance of 295ft is provided.Increased obstacle clearnce will be provided in moutaneous areas.

-----For NOn precision Approaches:

MOC for the initial segment:295ft without FAF on top of height of highest obstacle in the final approach and 246ft with a FAF.

-For circling,CAT C;MOC is 394ft above height of the heighest obstacle in circling area.

-----Precision Approaches:The margin is dependant upon aircraft approach speed,height loss and altimetry and is adjustable for steep glide paths and high level aerodromes(SEE how the designers think of everything...)

During the in intermediate app segment,the segment during which the A/C speed and config should be adjusted to prepare the A/C for final approach.For this reason the descent gradient is kept as low as possible during the intermediate approach,the obstacle clearance requirement reduces from 948ft to 492ft in the primary area(second area ends with no Obs clearnce so stay within the approach parameters)


Practical:

If you fly above moutaneous areas:refer to above how to calculate your true alt.
If you had a long flight ..make sure the QNH at arrival has been updated from thellast ATIS..a front may have passed rather fast and ATC may have been too busy to make the change.)
If you fly a non precision approach,check the highest obstacles,outside(missed appr)and on the final approach path.Use DME and other aids to ensure you are at the coreect height on the descent profile.Same applise to ils(even easier to use alts on approach plate)
On the ILS,you can use also a rough but on the safe side calculation: ils slope 3°(example),A/C 5 miles out from touch down:the A/C should be around 3x5x100=1500ft.

Hope this helps and if doesnt answer all questions feel free to ask..it makes me read more..
:ok:

Safe Flying,

M.85

[keithl]

Good Grief!!! To whom is this all addresed? I turned to "Altimetry and Obstacle clearance" because the topic interests me, but this looks like someone's homework.

To paraphrase an old saying, "If you can't swot quietly, like a gentleman, would you mind doing it on another frequency!"

[BOAC]

.85 - just before the good Captain closes THIS thread down, for your information!!:

YOU: BOAC..I have no clue how you found a 2000ft difference blah blah blah(What do you fly??)

ME: enroute MSAs which can be in error by over 2000' in extreme cases

I fly 737s over the Alps, mostly, with MSAs of 18000' or so. At ISA -20, there is a TEC of 1440'. At ISA -30, 2160'. That's how. Now try the Himalayas. Not to forget the wind correction also.

Oxford should have covered this:confused:

NB: A good idea to click on 'Preview Reply' before posting!

[M.85]

hy hy SIR!:hmm: :ouch: :ouch:

M.85

[alf5071h]

Before Capt Stable closes this thread … …

Re the previous thread on temperature error correction; why moderate with an axe by ending a thread after a minor tiff. The discussion reflected most flight safety or CRM processes; the need to exchange safety information and gain understanding of the problems. Not withstanding that the thread subject was covered before, the information requested was either not immediately available or the new request was indicative of the high personnel turnover within our industry (including expanding Pprune membership). One should not immediately conclude that the questioner has screwed up.
If you conclude that pilot error was the cause of an accident, then start the investigation again.
I too followed the instructions given for searching the website; initially I made an assumption / error that the subject would be found in Safety and CRM, or that it would be called temperature correction; nothing of course was found. Similarly nothing was found of relevance from a full search.

The point here is that we all should consider the other persons point of view; how do they see a problem, what is the question, what is their need or how can we assist. Good advice was given on the use of search, but this very powerful tool is open to error or the vagaries of how one thinks about the search subject (and not always in English). The need was specific - details of accidents. There are few good (meaningful) references to accident reports (thankfully few accidents or poor reporting). This is a common theme throughout flight safety and CRM, thus we should attempt to distribute the very best information there is, even if it means reiteration; some of us need to hear it two or three times before the message sinks in.

This situation, IMHO, also indicates the need for a repository of good reference material. In the S&CRM section alone, reference has been made to a wealth of CRM or safety material available from a range of sources; Pprune has excellent links, but often the material at the link is regulatory or impractical. The value in the Pprune interchange is to relate experiences and lessons learned at the working level; those who have yet to interpret the academic papers and put the good safety points into practice need this forum to gain that vital safety information. Whilst this forum is not yet a permanent repository for documents or pictures, let us continue to exchange information meaningfully.

Have stability in moderation, but relaxed stability in flight gives greater maneuverability without degrading safety.

[OzExpat]

MOC for the initial segment:295ft without FAF on top of height of highest obstacle in the final approach and 246ft with a FAF.
This was probably just a slip of the fingers on the keyboard, but the first part of this statement should read : "MOC for the final segment".


the obstacle clearance requirement reduces from 948ft to 492ft
This one is undoubtedly also a slip of the fingers... initial segment MOC is 984ft, reducing to 492ft., etc.


The minimum obstacle clearance equals 0 at DER
Not forgetting, of course, that the Obstacle Identification Surface (ie the 2.5% gradient) starts 5 feet above the elevation of the DER.

[M.85]

Dear al,

I wish my english style will one day be as good as yours and all pilots,including me,get your wisdom and open mind.Thank you for your inputs.

Oz,

Thank you for the corrections,and the 5ft error.Perfectionism is a good tool for professionalism.

To answer the BA capt,I still didnt get any info on the TEC...cant find any.Maybe British Airways ops could help him..or the CAA.A phone call is cheap nowadays.Hope he finds what he was looking for and shares it with us.

Safe flying,

M.85

[alf5071h]

RoboAlbert, et al,

The link for the full report into the MD-80 accident that I quoted in a previous thread is:
http://amelia.db.erau.edu/db/ntsbaar.htm#96
Select year 1996 and report number AAR96-05; I suggest that you right click and use Save Target As, the file AAR96-05.pdf size is 752 kb.

The report is well worth reading for many safety issues other than altimeter error; NPA, CANPA, descent below MDA, human factors, and chart design all feature.