Hornet C radar Altimeter Questions

[drtbkj]

Good Day, All,
I’m taking advantage of a slow work day to seek answers to a couple of altimeter questions…
Flying the Hornet C off a carrier, I leave the radar alt. on standby until I’m on the Cat., then I go active. I get a reading of 10 feet, but…
Question1- Isn’t the radar alt held in stby when WOW? So I therefore not get a reading?
Next, I consistently notice that the Radar alt. value is _higher_then the Baro.alt. , even over water. So…
Question 2- how is that possible? Radar/baro alt. should be the same over water, and radar should be lower the Baro. over land, should they not.
Off the carrier is set the altimeter at 3059.On deck it shows the correct 60 feet.

[Malc]

Simple answer - Radar altimeters aren’t foolproof, and the higher you go the less accurate they can become.

Tidal, electromagnetic, ionospheric, and tropospheric conditions all play a part in their accuracy, and while I doubt all these are modelled in BMS, the error between the two (radar vs barometric) simulate well the overall effect.

[white_fang]

Well in one case radar alt. can be higher over sea, where on negative amplitude of waves

But in other case it is probably BMS :evil:

[Frederf]

BMS default weather is pretty bad so barometric altimeter is way off due to lapse rate of temperature being unlike the real planet. Get real or generated weather and it’s much closer.

If baro and radio altimeter disagree, check against orbit view info bar true altitude. You should find that radio altimeter is much closer.

[drtbkj]

Thx, Guys. It’s interesting. Malc says radar alt is suspect, but Frederf about weather in BMS. While I respect both, I have wondered. It has always seemed to me to be asking a lot of BMS to deal with weather.
So, I would say the best bet is to go with radar.
One more thing, regarding the WOW question- what is the proper time to take the radar alt. out of standby?

[Dee-Jay]

@drtbkj:

Question1- Isn’t the radar alt held in stby when WOW? So I therefore not get a reading?

Question 2- how is that possible? Radar/baro alt. should be the same over water, and radar should be lower the Baro. over land, should they not.

1 - No.
2 - What is the local pressure / temperature / what is your altimeter settings ?

http://www.flightlearnings.com/2010/04/15/altimeter-part-two-principles-of-operation/

This indicated altitude is correct, however, only when the sea level barometric pressure is standard (29.92 “Hg), the sea level free air temperature is standard (+15 degrees Celsius (°C) or 59 degrees Fahrenheit (°F)

[drtbkj]

@Dee-Jay:

1 - No.
2 - What is the local pressure / temperature / what is your altimeter settings ?

http://www.flightlearnings.com/2010/04/15/altimeter-part-two-principles-of-operation/

This indicated altitude is correct, however, only when the sea level barometric pressure is standard (29.92 “Hg), the sea level free air temperature is standard (+15 degrees Celsius (°C) or 59 degrees Fahrenheit (°F)

Dee- Jay, 3059 is what the tower always gives me, what what I set in the Kollsman Window(see , I did read the link- ).
You know, I can understand air pressure being different at different points. I can see instrument fallibility. What I wonder about is while the amount of the difference between radio and baro varies, why is radio alt. always higher? Shouldn’t they match over water at least occasionally? That’s what made me wonder if I was doing something procedurally to cause this.
As for the WOW question, would switching radio alt. to active on the CAT be good procedure, or is there a better way?

[Stevie]

RADALT over water is going to fluctuate, as it gains return off wave crests and valleys…personally, I’m not sure just how much faith you can give RADALT over water in conjunction with sea state - there has to some sort of correlation, I’d think. Otherwise - I’ve seen AV8B RADALT dead nutz to about 10 feet AGL over level ground, and I’d think the Hornet should be the same.

Also - if you read the book (the NATOPS) you’ll find that Hornet RADALT is not displayed above 5K AGL. I’m figuring MVs in BMS…

[Frederf]

Up to 29,990’ in the training navigation TE. Orbit view says 29,991’. AR says 29,990. Baro says 29,280’. That is on a custom weather (A 29.84"). BMS default fair: true 29981, rdr 29980, baro 28710. Ah here it is the sunny weather 30.59" altimeter winds 360 @ 4. 29986 true, 30000 rdr, 28,500’ baro.

As you can see BMS radio altimeter is dead nuts over a sea surface. It’s the baro that’s wrong. Go fly formation with a tanker at 24,000.0000000’ and have a look at your HUD tape.

Radio altimeter is a radiation health hazard. I could see a naval aircraft having a WOW interlock as it’s expected for people to be under it right up until launch. I don’t think F-16 suspends radalt with WOW. You definitely want it immediately on flight so ready to go at or slightly before getting in the cat makes the most sense to me. I would be very surprised if it’s a post-launch item.

[Stevie]

I don’t think the Hornet strangles it’s RADALT WOW either…but one can turn it off. But in reality, there’s far more hazard from the ship’s radar(s) than there is from the aircraft’s RADALT emissions.

[drtbkj]

Thanks, Guys. I can understand all the variables involved, what remains weird to me is that every time I think about checking , radio alt is approx. 1100ft higher.