Sunday, June 26, 2005

Is It Hot In Here?

A day spent with ice packs on my spine seems a good one to talk about heaters.

A pilot I know flying this airplane in a moderate climate confessed to not understanding how the heating and ventilation system works, and I think that's common, as the chief pilot had simply decreed "don't touch this control at all, and only adjust this one if you really have to." Lets see what's so complicated.

The heating system and the ventilation system use most of the same ducts, and I'll start with ventilation. Early models--and remember most of them are still flying--had separate ram air scoops for the passengers and for the pilots, but later models had one bigger scoop for both. Passengers can adjust gasper outlets (those little twist-to-control air blowing thingies) above their seats, and according to the manual the pilots don't control the flow of air reaching the gasper outlets. Control freaks that we are, we could increase the airflow by putting the airplane into a high speed descent, but we'd rather not.

I can see from the schematic that there is one more way we might affect cabin air flow, at least in the later models with the shared ram air scoop. The duct just inside the scoop divides into two channels, one of which goes straight to the cabin gasper outlets while the other leads to the manually controlled ram air valve, controlled with a knob by the captain's right ankle. In the up position, the valve is open and ram air is being freely admitted to the heating and ventialtion system. Lowering the knob closes the valve until in the fully down position no ram air is going to the heating/ventilation. You'd think in that case more air would be available to the gasper outlets. Perhaps the airflow is already so great it makes no difference. I'll have to experiment on hapless passengers some time and find out.

Whatever it does to the gasper outlet supply, the ram air valve admits air to a space (called a silencer plenum) under the cockpit floor from where it can go to the pilots' footwarmers, the windshield heater outlets, and the cabin air control valve. If the cabin air control valve is open, then the air passes through it to the cabin baseboard outlets.

The knob for the cabin air control valve is inconveniently located on the floor behind the co-pilot's seat. This was the "never touch" knob mentioned above, and from the location it seems like the manufacturer didn't want us messing with it either. Pull it up to close it, and down to open it. (You didn't think it was going to function in the same direction as the ram air valve knob, did you?) So the cockpit and the cabin are sharing ventilation air. The manual recommends than in hot weather we fully close the cabin air control vent, keepig all the ventilation air for ourselves. We are, after all flying the airplane, and all you're doing is looking out the window or reading the paper. In case we feel guilty for depriving you of ventilation air, the manufacturer assures us that even if we shared it, the low velocity air exiting the baseboard outlets wouldn't cool you much. We also have little electric fans blowing air on us, and we can open our windows.

The same model number that combined the passenger and crew ram air scoops added an electric fan (20A L DC bus) inside the air duct to provide cabin ventilation on the ground. It should be turned off in flight.

The heat for the heating system comes from the hot bleed air takes out of the engines at station 3. It is routed from each forward engine nacelle, and up through check valves to a common duct in the cabin roof. From there bleed air intended for heating is routed forward along the left side of the fusilage, through the bulkhead that separates the cabin from the cockpit and up to an electrically controlled (5A L DC bus) valve under the cockpit floor. From there it may be injected into the heating and ventilation system, just after the ram air valve.

That is all relatively simple. Now we get to the heating controls. The heater has three modes: OFF, MANUAL and AUTO. The OFF mode closes the cabin heat valve and supplies no heat to the system, but if contamination of the bleed air is suspected, the bleed air switches should be selected off too.

The MANUAL mode allows the pilot to operate another three position switch marked MANUAL WARM, HOLD, and MANUAL COOL. The switch is spring-loaded to the HOLD position and gives the pilot direct control over the setting of the cabin heat valve. It takes about 30 seconds for the valve to motor between fully closed and fully open, so presumably with practice you can turn the the heater from OFF to MANUAL, then hold the switch down to MANUAL WARM counting "one Mississauga, two Mississauga ..." until it reaches the desired openness. It would take quite a bit of practice, because the temperature thus produced depends on your engine speed, outside air temperature, aircraft speed and ram air lever position.

The AUTO setting is supposed to maintain a constant cabin temperature in flight, despite all those variables. Why does the MANUAL setting exist, then? I told you: pilots are control freaks. (Did I mention that it bugs the heck out of me that since the last Blogger upgrade a space is enforced between the end of a blog entry and the byline? Damnit, if I wanted a space I would have put one there.) We'd rather be too hot or too cold than not be able to control a valve ourselves. The automatic heating control works a lot like your house thermostat, except that the dial is just labeelled COOL and WARM with a little arced arrow between, rather than having exact temperatures marked on it. A temperature sensor, in the back of the cabin in early models and at the front in later models monitors the temperature. There's a little fan mounted over the forward temperature sensor in order to prevent air stagnation around it. The outside air temperature, cabin temperature, and silencer plenum temperature are compared via a Wheatstone bridge--hey I remember that from highschool physics, never thought I'd care about it ever again--to the temperature selected on the dial.

I'll save malfunctions related to this systems for another day.

If you're ever a passenger on an airplane small enough to have no flight attendants (as this one is) and you are too cold or too hot, wait until one of the pilots looks back into the cabin--we're supposed to from time to time to make sure you're not rioting or having an orgy--and make eye contact then signal your discomfort with either mimed shivering or exaggerated fanning yourself. If we're not completely clueless we'll try to do something about it. Another tactic is to write a little note and pass it up to the front row. In this day and age you have to be extremely careful that "tell pilots it is too cold in the cabin" isn't misread as "tell pilots I have a bomb in my carry-on" so please use neat penmanship.


aasmodeus said...

Wow. I'm a technical person and enjoy your blog in general, but I confess I started to glaze over at para #10 or so. I never realized it was that complicated (for a lay person). So, I caught the 20A fan, but is that really what's powering the gaspers on the ground, or is that something else entirely? I can't imagine a 20A fan controlling all 100+ gaspers at the rate they flow even when on the ground. My son likes to play with those a lot, so I am intimately knowledgable about their capacity :)

Aviatrix said...

On the ground in this airplane, the gasper outlets would provide a very weak stream of air. Air conditioning is an option, and it's quite heavy, so not usually installed. If it was a hot day, and there was a delay on the ground, you would be quite hot inside, and very grateful for the cooling airflow after takeoff.

I think the motive force for the gasper airflow in a pressurized airplane is bleed air pressure, which would also be lower on the ground. Maybe there is a compressor driven by the APU. Here's a recent blog entry by Dave, describing the conditions in a pressurized aircraft when the APU is unserviceable.

Aviatrix said...

Actually, come to think of it, the duct divides for the gasper outlets and the baseboard outlets upstream of the fan, so the airflow to the gaspers ont he ground would be virtually non-existant.

aasmodeus said...

Ahhhhh... I only assumed commercial airliners (which is all I've ever flown except for a puddle jumper in the Caribbeans), and therefore always pressurized. Thanks for the clarification.

dave said...

Our gaspers are driven by cabin fans, pack turbines, and/or the ground air hose. Good blog by the way.