Helmet,
Much of the discussion above has mentioned rig positions and moving rigs as a danger. I am assuming the concern here is if the aircraft does not have radar, or am I missing something? How many accidents have occurred with CFI unseen rigs V CFIT? Is it a biggie?
Yes, you are missing something. The whole point about the 'enhanced' function is that it has to have all obstacles mapped - without that this part of the EGPWS itself becomes a hazard for offshore operations. Anyone who has operated offshore will know that mobiles can arrive with little notice, when used as accommodation they can pull off to the standby position in bad weather, large derrick barges (some of them with a vertrical extent of 500ft) move around the patch during construction phases and ships are constantly moving in and out of the area.
There have been proposals for compulsory registration of vessel movement but it really is a non-starter. Operator entry of positions into a data-base is frought with danger and will never be accepted by the regulator let alone pass the Risk Assessment process.
Yes, most helicopters, which operate IFR offshore, do carry radar and it is used to map the obstacle environment for the ARA. When this is used in conjunction with SBAS or GBAS systems (or whatever area nav system you are using at present), it can provide obstacle free approach sectors.
The other fact that must not be ignored is that the landing site that you are approaching, sits (on a high number of occasions) on the highest obstacle in the area. For those reasons the 'enhanced function' should not be used for obstacle avoidance until and unless it can be further enhanced with a dynamic obstacle mapping function - preferably more advanced than the weather radar and certificated for purpose. If use of the 'enhanced' function is set aside for offshore operations (as it should be until we solve the above problems), we are left with the GPWS elements.
Without - for the time being - resurrecting the discussion on the specific purpose of the Modes which was covered extremely well by Nick and Helicomparitor (what a pity those two can't work together), it occurs to me that the GPWS could be improved if tailored for offshore operations. As anyone who has read the technical description will confirm, reading the description of the Modes takes a great deal of time and concentration because it is lacking in the basic description of the 'protection scenarios' (not my phrase but one that has been used by the RTCA HTAWS working group); thus we have not only to understand the function/purpose off the Modes but apply them to (in the case being discussed) offshore operations.
That the radalt/AVAD combination has worked (reasonably well) for so long results from the less complex obstacle environment offshore and the presence of the weather radar. The offshore environment - at least in those areas in which I have operated - has an obstacle vertical limit of 500ft and a level floor; consequently there is no danger of rising or a falling surface and no obstacles above that height.
The manufacturer's preset height is set to 100ft; this is well thought out because descents to within 100ft of the sea is an unusual occurrence with the heights of platforms in, for example, the North Sea. This means that it can be used as a safety net call which results in some action by the crew. (The problem with such a simplistic system is that the 'call' occurs on every approach to a runway/heliport which might be regarded as a nuisance.) It can also cause a problem when fitted on a SAR machine which operate routinely at those heights; however, I am told there is a muting function for those.
That means that I can add another element to offshore operations; although the floor is sea level, the helicopter never descends to that height (unlike onshore operations). Does this affect the logic of the EGPWS - probably not but the switching from takeoff to approach mode does use a combination of height and speed (and other parameters) so it needs to be considered.
The helicopter also takes off and lands above the floor. Does this have an effect, well I'm not sure but I do know that at least one type that I have flown (still in use) the power is limited to the extent that a take-off manoeuvre could (in low winds) result in a descent below the takeoff site while accelerating to Vy; the same might occur if a go-around is initiated for any reason. Does this have an effect; once again I am not sure but would like to know because such manoeuvres are often conducted at night.
I therefore conclude that whilst we have the potential of fitting equipment which can enhance safety - particularly in onshore operations; there does need to be an examination of the specific operating environment and the required 'protection modes' so we can ensure that it is fit for purpose in all environments. Good news then that the RTCA HTAWS have been formed - let's hope that they consider the concept of operations (CONOPS) in as much detail as RTCA-196 did for NVIS.
Sadly, and to return to the initial purpose of this post, I do not see the database function being part of those operations in its present form.
Jim