Flying quadrantals but without 1013 set

[marsman]

Do peeps fly quads without having 1013 set?? Wouldnt setting 1013 in some cases be dangerous if you only have the one alt in the aircraft? Say if the pressure was higher than 1013.

What do you guys do if you have controlled airspace directly above ..say the LTMA- and high terrain below you. How can you still fly them?

[High Wing Drifter]

Marsman,

If the QNH is higher than 1013, then the FL30 will higher than 3000'. The problem is if the QNH is lower than 1013, then the lowest usable FL for quad increases. Meaning that for a say a 360-089 track you would need to be at FL50 rather than FL30.

If you fly a quad on QNH then you are just at some arbitary FL.

In the case of the LTMA, you will only be at QNH below it so there is no opportunity to use the quad.

[DFC]

Do peeps fly quads without having 1013 set?? Wouldnt setting 1013 in some cases be dangerous if you only have the one alt in the aircraft? Say if the pressure was higher than 1013.

What do you guys do if you have controlled airspace directly above ..say the LTMA- and high terrain below you. How can you still fly them?

Since the transition level under the London TMA is 6000ft, one would not use Standard Setting unless going above that altitude. Above 6000ft keeps you above the highest terrain in the UK.

Since quadrantals are designed to be used above the transition altitude, you would not use them below that altitude.

If above the transition altitude, trying to fly a quadrantal on the QNH or Regional pressure setting defeats the whole idea of a system of levels referenced to 1013.

When flying in say NW Wales where the Transition Altitude is 3000ft but the Terrain is above that level, there is a handy chart in the AIP where you can enter with QNH and Minimum Safe Altitude to find the minimum safe Flight level. After that simply select the appropriate FL for track which is at or above the minimum.

Regards,

DFC

[ShyTorque]

Marsman,

Quadrantals are based on the Standard Pressure Setting (SPS) of 1013 and don't begin until the transition level, always above 3000 feet and varying according to the prevailing barometric pressure on the day or by mandate in some areas. A "quadrantal" isn't relevant unless 1013 is set.

The idea of the quadrantal rule is to give at least 500 feet separation between aircraft on converging headings. Obviously, an altimeter setting at variance from the SPS gives a difference of 30 feet per millibar, which could reduce (or increase) the separation from another aircraft using 1013, depending on the actual pressure subscale setting used.

With regard to your example, under the London TMA, it is not possible or relevant to fly quadrantals because the transition level is deemed to be at 6000 feet. Controlled airspace begins at 2400 feet amsl, so pilots set the London QNH, which allows terrain clearance using MSA, or a calculated 1000 feet above the nearest obstacle within 5 nm of the aircraft, if preferred.

If you check a chart, under the London TMA, MSA is, by no coincidence, 2400 feet, which allows a pilot to fly safely with legal minimum terrain clearance without entering the TMA. However, opposite way traffic may have no altitiude separation, which is why it's best to squawk mode C and speak to a radar unit for information on other traffic....

Further north, the Daventry CTA begins at FL45. Here, if you wish to fly a quadrantal (1013 set, of course), you must also remain below that FL unless you have a clearance to enter the airspace, which entails submitting a written flightplan, confirming it is accepted, then talking to the relevant control agency before entering the airspace.

So, consider a situation where you don't want to submit a written flightplan, but due to low cloud you need to comply with IFR when flying northwest bound, under the Daventry CTA.

As stated, the quadrantal rule requires you to fly at FL45, except that would put you in controlled airspace, without a clearance to enter, not legal. So, to ensure you have adequate terrain clearance you should use the regional pressure setting (or a local QNH if flying within 25 nm of the airfield that issued it) on one altimeter.

In this example, if you fly within 5nm of the Rugby radio masts (1188 feet amsl) you should fly a minimum of 2188 feet, call it 2200 feet for cash, using either the Barnsley Regional Pressure Setting, or a more accurate local QNH from Coventry or Birmingham, both those airfields being within 25 nms of the obstruction. If you wanted to go higher, you could set 1013 and fly a maximum altitude equivalent to “FL44”. (I might choose to do this if it put me in clear air above a layer of cloud). This would give 400 feet separation from an aircraft flying southwest at the correct quadrantal, FL40.

One must beware of low regional pressure settings (i.e. less than 1013) when flying under controlled airspace designated by a flight level. Although you might appear to be remaining below on QNH, you might actually be inside the base of the airspace so plan carefully and set 1013 when climbing in these circumstances.