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zudhir
2nd Nov 2011, 15:11
I need a clarification on helideck t- Values.

Is it legal to land a 6.8 ton certificated machine on a 6 ton offshore helideck if the AUW at the time of landing is less than 6 tons?

If yes, why is it not in contravention of CAP 437? Is there any guidance on this other than CAP 437?

Thanks.

JimL
2nd Nov 2011, 15:40
It really depends upon the Authority - although some States adhere to the static and dynamic stress loads provided in Section 1.3.2 "Structural Design" of the ICAO Heliport Manual (Doc 9261), not all do. This will condition the loading data that has been established.

For example: most of the older North Sea platforms were built for the S61 (using this method) but, following a safety study (comparing the likely 'dymamic' loading following an engine failure on approach or departure), they were subsequently approved for use by the EH101 (and others).

Practically speaking, there is nothing wrong with landing a helicopter with a landing mass of 6t on a deck that has been approved for that mass. Perhaps more importantly, is the D size sufficient for that type of helicopter?

The responsibility for offshore operations and, in particular, whether a deck is suitable for the type of helicopter being used, rests with the Operator. Each deck should be authorised by the Operator as part of the safety program (SMS). An individual pilot should not be in the position where he has to make such a determination.

Jim

Ikoyian
3rd Nov 2011, 15:41
I suspect you are considering the AW139 when you mention 6.8 tonnes? I have known decks to be given an approval for the 139 to utilise a 5.4 Tonne deck previously used for the Bell 212.
To give approval for this type of deck to be used, the deck and structure has to have a recent structural inspection certificate and dispensation from the local regulator. The regulator will then base their decision on the performance of the aircraft type and the likelihood of a heavy landing occurrence and the deck having a sound structure. As previously mentioned, the S92 operates to the 9.3 tonne S61 decks that have similar considerations.
A number of AW139 aircraft are operating to the old B212 decks as they fit under the B212 D code.
Further info can be found in ICAO annex 14 but does not mention the above dispensations. Gulf Helicopters have dispensation for their 139 fleet to operate to some decks so they might be able to help you with the process.
There is no dispensation for smaller D coded decks, I think you know that.
Hope that helps.

212man
4th Nov 2011, 05:11
I think posters are reading this the wrong way round - the original question is can you operate an aircraft with an MCTOM that is greater than the 't' value, if you are actually at an AUM below that 't' value?

Logic says of course, but it probably depends on exactly what the 't' value means. Is it the maximum mass of an (any) aircraft the deck is designed to accept, or the MCTOM of the specific aircraft for which the maximum 'D' value used is? CAP437 isn't very specific.

JimL
4th Nov 2011, 08:21
In fact, the 't' value is representative of an estimated landing weight (i.e. a force) not a mass. Examination of the (heliport manual) text referenced indicate that the 't' value should be based upon the worse conditions derived from consideration of the following two cases:
1.3.2.3 - the helicopter on landing; and

1.3.2.4 - the helicopter at rest.
I will not bore you with a repetition of the text contained in 'ICAO Doc 6261 Heliport Manual' but in the first section are included:
Dynamic load due to an impact on touchdown (which considers vertical speed at landing - both for the normal and emergency cases);

Increased loading on the FATO due to snow etc.;

Wind loading;

Punching shear (taking into account the likely loading at the point of contact - e.g. the wheels/skids);

The loading of the helicopter at rest also contains a number of elements - all of which are fairly logical.

Although there is debate on the subject, design of any helideck/heliport is usually based upon a 'design' helicopter, which is assumed to include the likely elements of the population of helicopters that are expected to use the facility. Authorisation has to be based upon the MCTOM of each helicopter because that is the only way that all expected masses can be considered.

Although the 'D' value represents an absolute limit - because it is based upon a invariable constant, the same is not true for 't' value, because of the assumptions made in the design process. In view of this, there is scope for States to conduct a safety case to consider helicopters with masses that exceed the original design 'mass' value. These studies are usually based upon installed power and assumptions made about the likely vertical speed of a helicopter that has had an engine failure on landing.

Unfortunately, these calculations are based upon the values that have been demonstrated during flight testing by the manufacturer - which are likely to be the Category A procedures. These may or may not be representative of the values that could be seen during an approach because they are likely to be outside the testing regime (exposure).

As with all of these questions, first recourse should be the the Operator - who should then explain the policy to the pilot. If there is doubt after this exchange, then an Occurrence Report might be filed (but this is quite a heavy tool to use).

Jim

Geoffersincornwall
4th Nov 2011, 09:18
When I communicate online with the UK CAA they respond with a kind of 'thank you for your mail' message and also remind me that new EASA legislation next year will make the pilot personally responsible for obeying all laws.

Does Jim's suggestion that the first port of call when faced with this problem (being asked to land at mass greater than the 't' value by the customer, via of course, the operator's operations system) should be the operator, would the pilot be indemnified under these new laws? Or would he be 'personally liable'?

WIHIH???

G.

212man
4th Nov 2011, 11:43
EASA legislation next year will make the pilot personally responsible for obeying all laws.

I doubt that changes much from the current situation?

Geoffersincornwall
4th Nov 2011, 11:58
Why the change in the law then I wonder? What are they getting at?

G.

squib66
4th Nov 2011, 16:40
JimL

If the t value is based on landing weight, and that weight needs to allow for emergency cases, something doesn't seem right.

Typically what g is anticipated in the sorts of emergencies envisaged?

Geoffersincornwall
4th Nov 2011, 17:26
If I understand Jim correctly he is saying that the only data available is that based on Cat A operations but this is problematic given that operations are generally conducted 'with exposure', meaning that the helicopter is not operating to Cat A. Thus there is an anomaly between theory and practice and the rates of descent involved should a power plant fail are therefore likely to be greater than expected.

G.

JimL
4th Nov 2011, 19:10
Hi squibb,

What I was attempting to explain was that the mass value ('t') is the visible limit (painted on the deck); the dynamic loads due to impact on touchdown (weights) used during the design process are: the normal touchdown at 1.5 x the mass ('t') - which equates to a ROD of 6ft/sec; and the emergency touchdown at 2.5 x the mass ('t') - which equates to a ROD of 12ft/sec. These are then conditioned by the other factors.

Geoff is absolutely correct in his comments on my interpretation of the safety studies (which are only used if approval is sought to accept a helicopter above the original design MCTOM).

Jim

Manchester
2nd Jan 2012, 17:00
While not answering the original question, the basis of the operators’ safety cases mentioned by JimL for “over-t” landings (eg S92s on S61 decks) appears to be at:

Page 5 Article about the function of an Aviation Expert Witness (http://www.theaviationexpertwitness.com/Page_5___Article.html)

in ARTICLE 2: HOW TO LAND A 14,600kg HELICOPTER ON A 9300kg-RATED HELIDECK