Regional Jet 700 ER/LR (CRJ700)
Flight Crew Air Transport Pilot (ATP)
No Specific Anomaly Occurred All Types
General None Reported / Taken
Human Factors; Procedure; Weather
On the ENDEE Arrival into MDW we were cleared to descend via. There is a crossing restriction to cross NOGEE at or below FL180. On the descent we were on the snowflake and complied and were below FL180 per the profile at NOGEE. However; the altimeter setting was approximately 30.50 so once crossing NOGEE; we set in the altimeter setting and the altimeter spun above 18;000 FT briefly. We continued the arrival as instructed.I do not believe we erred here. The instruction was to cross at or below FL180. Flight levels are predicated on a standard ALT setting of 29.92; so you are expected (based on my view) to cross that fix at or below 18;000 with a 29.92 setting. That is exactly what we did; we crossed [at or] below FL180. In my opinion; however; this is a problematic OPD and I see it creating problems. Having a crossing restriction at the transition altitude is a threat (again; in my opinion) and creates a potential for deviations. If the altimeter is set; for example; too early while conducting the checklist and the local ALT is low (i.e. 29.60 for example) then the crew could potentially cross high when they believe they are on profile. In our case; when you wait to set it until after crossing the fix and going below FL180; if you wait too late; you will set yourself up for failure on the next fix which is to cross KOOKS at or below 16;000 (which must be done with a local setting; so if the ALT is very high and you delay in setting the ALT; you have a lot of ALT to make up in a short period of time). It may be prudent to consider revising the crossing restrictions on this OPD to mitigate issues. Given the above explanation; it is possible this is setting up for problems.
CRJ-700 Captain questions the advisability of using FL180 for crossing restrictions on RNAV STARS due to the impending change to QNH. An early change over may cause the restriction to be missed and a late change over may cause problems with the next restriction.