Baro-altimeter Variations Due To Temperature Gradients

[Eridanus]

Hello all,

I am an Aeronautic angineer, I work at implementing and updating flight simulators. Recently I have had a query about a simulator not sensing altimeter variations due to temperature changes. I have been making some figures and considerations but I would like to listen to the opinion of an expert if possible. Let's put it this way. A comercial aircraft flying an horizontal trayectory (for simplification) may encounter temperature gradients. If the temperature gradient is associated to a pressure system (high / low) indeed the altimeter will sense this variations. The standard atmosphere model, (troposphere or estratosphere) will agree with this, provided we modify things at sea level (pressure and temperature). To be more precise, if we modify the temperature at sea level, on this model, we will have a modiffication on the pressure distribution. The barometric altimeter is a device which meassures pressure changes so it will sense this. My concern is that in real life ... how intense can be a temperature gradient of this kind... meaning, can it really take place in a matter of a few tenths of kilometres? and in which magnitude? is so, then, agree, you will see the altimeter moving. But I thinks this hanges are really slow, even for an aircraft flying at 900 kms/hr. On the other hand, the instructor can modify the temperature at sea level, in a matter of seconds. This would cause the same effect, but the simulator manufactorer has prevented this. My guess is that (after having done a few figures with the ISA model) a change on something like 5 deg Celcius at sea level , would imply a change in the altimeter of thousand of feet. This would be very unusefull for training purposes.

The entity making the complain argued that this is required to simulate the convective hot wake of a volcano ( this inded would imply a severe temperature gradient in a small distance) but then, if I am not wrong (please correct me if so) the pressure gradient across the boundary of such a wake is null, therefore the altimeter wouldnt sense this (not to mention the problems of pitot blockage and so forth).

Please if you can contritubute to throw some light on this matter, do so.

Best regards

Eridanus