Today’s blog post was written by a fellow pilot, Pat Flannigan, who is a professional airline pilot, aviation blogger, and certified flight instructor (CFI). Pat has many years of flying experience in his log book and writes the blog, www.AviationChatter.com. You really need to check it out.
Well the other day he asked if I would be willing to post an article he wrote on “The Hidden CRJ200 Altimeter and Airspeed Indicator.” It is a relevant article enlight of Air France 447 accident.
So here is Pat’s article. I hope you enjoy it!
_________________________
The Hidden CRJ200 Altimeter and Airspeed Indicators
by Pat Flannigan (AviationChatter.com)
All the speculation over Air France 447 and the probable pitot-static failure presents an interesting scenario. Suppose you have lost all pitot-static systems in the CRJ: you have no airspeed and no altitude indications. To make matters worse, the weather is standard simulator conditions: IMC down to minimums anywhere you go. The question is, how do you safely descend and land? In the CRJ, there is a way — albeit a bit unorthodox.
In this scenario, the first challenge to aircraft control comes with speed management – the very thing that may have brought down Air France 447. We can initially fly pitch and power, which ought to give a reasonable margin of safety. But there is another speed indication. The GPS on the CRJ provides groundspeed information, which isn’t terribly accurate but does provide a good ballpark figure for indicated airspeed.
Of course, we have to keep in mind the two dominant factors contributing to the difference between groundspeed and indicated airspeed: wind and altitude. Recall that groundspeed is true airspeed +/- tailwind/headwind component, and that true airspeed increases with altitude. As we descend, the groundspeed indication becomes increasingly accurate. And if the wind speed is known, we can even estimate our actual indicated airspeed with great precision – though I would suggest applying a generous margin of safety.
With airspeed under control, we need to determine our altitude. Believe it or not, the CRJ-200 does not indicate GPS altitude to the pilots, and ATC can’t help because the transponder simply broadcasts altitude from the busted left or right altimeters. The radar altimeter ought to work well, but only indicates height above terrain from 2,500 feet on down. But there is a hidden altimeter – and it’s staring you right in the face.
The CRJ features an overly complicated sounding unit called the cabin pressure acquisition module (CPAM), which is a fancy altimeter in the cabin. It provides some much needed information on EFIS Display 2 – particularly the cabin altitude and cabin pressure differential.
If the airplane were completely depressurized (with a cabin pressure differential of 0 (zero)), cabin altitude would equal pressure altitude. We have a working altimeter again! Of course, we have no idea how high we are, and it would be a bad thing to pop the EMER DEPRESS switchlight and blow up your passenger’s eardrums for no good reason.
I would suggest switching the cabin pressure controller (CPC) to manual, spinning the rate knob to minimum and manually climbing the cabin to a cabin altitude of about 10,000 feet or 0 (zero) delta p, whichever is first.
If you hit 0 (zero) delta p, then you can press the EMER DEPRESS without fear, using the cabin altitude readout as a makeshift altimeter. If not, then simply begin a controlled descent while manually climbing the cabin towards 0 differential.
To be extra safe on the approach, make sure you get an ILS to the longest runway and land with plenty of speed to spare – watching for abnormal pitch and power settings for the approach.
As with any “outside the box” aviating, use these procedures at your own risk. You certainly won’t find this in any company manuals or in the Bombardier QRH. Share your thoughts or alternate solutions to this scenario in the comments below.
_________________________
So, you see, there is a reason why you learn all that stuff in ground school for your private pilot certificate and your commercial certificate and your instrument rating. Without a thorough foundation, this article might as well be written in another language.
To Your Flying Success,
Jeffrey