Mutt,
A very interesting question which will always polarise opinions.
Most operators with whom I am familiar match the two altitudes for simplicity. There are all sorts of "after-market" performance reasons offered for using a single FRA, the vast majority of which are specious - the only sensible reason is simplicity.
Depending upon the aeroplane and the departure path, combining the two altitudes may not only be inefficient but also counter-productive in terms of mindsets and flightpath management.
The reason we have special departure/emergency escape/OEI procedures is to reconfigure the aeroplane as quickly as possible to the clean enroute climb configuration within the engine limitations without hitting the obstacles. We plan for the worst case, ie an engine failure at Vef at the maximum weight that satisfies all of the rules. That, by definition, will create the lowest safe flight path to be flown. Therefore, every second that elapses after Vef without an engine failure is money in the bank because, despite what Empty Cruise posited, the OEI profile is based on minimum obstacle clearance and a level acceleration and thus any AEO profile will normally be well above the OEI profile. The only way that the OEI profile could be compromised is if the pilot chose to accelerate at a lower gradient than the OEI second segment, which is not only stupid but illegal in some jurisdictions (eg Oz, which I thought JT would mention).
So now we need to look at simplicity versus efficiency.
The best flap configuration to be in at the time of failure is clean, followed closely by the lowest flap setting authorised for take-off. The AEO FRA is determined largely by legal minimums plus the time and obstacle clearance factors that allow for human error at a critical phase of flight. Having achieved AEO FRA without a failure, accelerating at a gradient not below the OEI gradient to then achieve a better flap configuration is not only safe but also offers the prospect of even better climb/acceleration performance in the event of a subsequent failure. That is potentially even more "money in the bank", easily verified by comparing climb gradient for each flap setting at the same weight and recognising that the potential climb performance will in most cases improve significantly as the speed increases beyond the relevant V2.
I accept the simplicity argument but I advocate the efficiency argument.
Now that I am old and grumpy, I really hate sitting through an AEO take-off at the worst possible flap setting that staggers up to a very high OEI FRA without configuration change "in the interest of simplicity" when I could have an even greater margin of safety by taking advantage of my AEO performance to optimise my configuration prior to any failure that may happen along.
For all of that Mutt, operational or organisational or national culture may end up being the safety driver for the policy decision, rather than efficiency. Best of luck....
You raised some later issues after I drafted my initial response above.
Raising the FRA above the 400' minimum does not always reduce payload - I presume you are always spoilt for runway length! Nonetheless, raising the minimum FRA is a safety-based commercial decision in that it buys time for the crews to get organised before accelerating either AEO or OEI. In many cases, there is a range of acceleration altitudes that will work within the engine limits, particularly for flex take-offs using thrusts settings below MCT. However, we don't tell the crews because there are too many variables and we want them to respect the published FRA as if there was little or no latitude for the acceleration.
Hudson's reference to at least one operator delaying the third segment level-off was, I suspect, highly context specific to very few destinations and related only to fire situations. The policy did not respect the FRA but did respect the OEM advice to carry out the initial fire drills in the second segment. My counter view was to delay the fire drill until starting the third segment - another great argument to be had over a few reds.
JT, in your (f) and (j) you mention elevating the FRA to solve the problem. I note that some Jepp models consider that the only option for dealing with distant obstacles. Because we plan all the way the MSA/LSALT to satisfy the Oz rules, we quite often look at a lower FRA to achieve a safe 4th segment climb before getting to the obstacle(s). For our overseas friends, the traditional Oz rule does not truncate the splay either laterally or longitudinally on the basis of a DR climb in IMC and turning can often not alleviate the problem. They have introduced an ICAO laterally truncated splay as an option but survey baselines are a major limitation for most jet aerodromes outside the major cities.
Stay Alive,