DOK001

Hi all, my name is Dominic, I'm a student studying an aviation degree aiming to one day fly commercial jets.

I was reading a discussion on various techniques used to measure airfield visibility that was placed on PPRuNe some time ago, which led me to ponder these next few questions.

Firstly, are all airfield visibility measurements made on the ground? If not, then can visibility just above the ground (Ie at decision height) be significantly different to that on the runway?

Secondly, I have read that a HUD / HGS allows an aircraft to land in lower visibility conditions by giving the pilot a localiser / glideslope image which superimposes on reality. If this is true, then the low visibility capability of the HGS would be dependent on the certification of the ground equipment. Can HUD's certify an aircraft to land in lower than normal visibility conditions at a non-ILS equipped airport?

Thirdly, how feasable are infra-red visual systems for commercial use? Do any airlines use them and if so are they commercially viable?

Lastly and hopefully an easy one, what is the actual NM distance measurement of Cat I, II and III levels of visibility?

Thanks in advance to all those who post a reply.

Cheers,

Dom

fatboy slim

In Europe:

The measurements for Runway Visual Range (which is different to Met Viz) are the numbers used for Lo-Viz approaches. Generally there are Touchdown, Mid-piont and Stop-end recordings so Air traffic will pass a figure such as "400, 300, 350" These are RVR readings in metres taken automatically at about a metre above ground level level by machines.

To answer your question about Viz being different at decision rather than on the runway, that is the whole point of RVR readings being measured, as it is the runway you have to land on and (for Cat1 and II) therefore have to be able to see. So whilst you may be able to see the runway fine from overhead or even 1000' it may be a pea-souper at runway level and RVRs tell you this.

The requirements for approaches are generally 550M for Cat 1, 300M for CatII and anything down to 0M for CatIII depending on the aircrafts and airline's rating A,Bor C. These RVRs can be increased to much higher figures for various reasons (terrain, lighting systems) on particular approaches.

I don't know anything about HUD except they look very fun.

Fragman88

Hi Dominic. I think the earliest civil HUDs were used on an aircraft called the Dassault Mercure (Like an early baby 737) by Air Inter in France for their Nuit Postale low vis procedures in the late 1960's.
After this, they seem to fall from favour in civil aviation although very much to the forfront in the military. I believe some of the later 737 NG versions may have the facility. but don't know much else.

A few years ago I did read that a couple of the larger US carriers were running a project to develop an integrated system involving HUD. At this time there was some concern over the longevity of ILS sytems due to frequency encroachment etc, and alternatives were being explored. The idea was to use GPS to get the A/C down to around a Cat 1 DH (around the 200ft/1/2 mile point), and then use a composite HUD for final guidance. The HUD would be fed from a variety of sources, low light TV, forward looking I/R, and millimetric radar( possibly using the leading edges as an enhanced aerial), all of these combined with radar reflectors, lights and thermal markers to delineate the runway.

I believe they had some success but ran into problems due to the fact that a lot of the equipment required was developed for, and in use, by the military and was still highly classified, and also , having expended vast amounts of their budgets developing this technology, the services were loath to give it away! I've heard nothing more since, but it sounded like a good way to go.

RVR used to be measured by a fireman counting visible runway lights, whilst standing on top of his fire engine (to give an approximation of flight deck height on landing). When transmissometers came into use, their measurement was factored to try to reflect the same measurement, but sometimes problems still arose in very shallow fog, with vis at 5ft being 50M but at 15ft, 10KM, but that's just aviation!

Hope this is of some assistance

Intruder

Alaksa Air has HUDs in some of their airplanes. I don't know any specifics of what additional capabilities or reduced minima they provide.

Luv 744s

More information about minima et al. regarding HGS (HUD) system capabilities including what was bought by Alaska Airlines can be found at:

http://www.awgnet.com/shownews/day1/hardwr10.htm

A quick summary:

Can fly Cat-IIIa ops down to 700 RVR and DH 50. Provides rollout guidance upon landing... and can provide takeoff guidance for as low as 300 RVR as well as lower landing minima at Type I ILS facilities, according to the article.

Total HGS system weight: about 60 pounds / 27 kgs for a cost of about USD $500,000.

Seems like at this time, such equipment would be real nice but likely more useful for routes that are frequently solid IFR, potentially with additional qualifying local conditions(?). That sounds like some of Alaska Airlines' routes in the less populated areas of Alaska such as arctic ops (Barrow, AK -- typically cloudy due to land/water thermal evaporation cycle).