10.4 Density Altitude, METAR/TAF and BoM Forecasts

Density Altitude

Density altitude (DA) is pressure altitude corrected for non-standard temperature — the altitude in the standard atmosphere at which the air density equals what the aircraft is actually experiencing. On a hot day, or at a high-elevation site, the air is less dense and the aircraft performs as though it were higher than the chart suggests.

Three things raise density altitude: high temperature (warm air is less dense), high elevation (lower pressure, thinner air), and low pressure (a low QNH or low-pressure system). High DA means reduced lift (less mass of air for the rotors), reduced thrust (less mass flow through the motors), reduced endurance (motors work harder, drawing more current), and lower maximum payload.

Worked Example

An RPA rated to carry a 1.5 kg payload at sea level on a standard 15°C day operates on a 38°C summer afternoon at an inland site 1,500 ft AMSL. Standard atmosphere temperature at 1,500 ft is about 12°C, so the site is 26°C hotter than ISA. Using the rule of thumb that density altitude rises about 120 ft for every degree above ISA, the density altitude is roughly 1,500 + (120 × 26) ≈ 4,600 ft; the lift and thrust available are reduced as though the aircraft were at 4,600 ft in the standard atmosphere, and the effective payload limit may drop well below 1.5 kg. A pilot who plans using the sea-level figure overloads the aircraft.

For most low-level coastal RPA work the DA effect is small, but it is not negligible on a hot day at an inland or elevated site. The practical habit: on a hot day, reduce the planned payload, allow more battery margin, and avoid the hottest part of the afternoon when DA peaks.

METAR — Meteorological Aerodrome Report

A METAR (Meteorological Aerodrome Report) is a routine, observed report of actual weather at an aerodrome, usually issued every 30 minutes. It tells you what is happening now, not what is forecast. A typical Australian METAR:

METAR YSSY 250230Z 18012G20KT 9999 SCT035 26/18 Q1018

Reading it group by group:

• METAR YSSY 250230Z — routine report for Sydney, dated 25th at 02:30 UTC.
• 18012G20KT — wind from 180° true at 12 kt, gusting 20 kt. The gust value is what to compare against the aircraft wind limit.
• 9999 — visibility 10 km or more.
• SCT035 — scattered cloud at 3,500 ft AGL; a low base can force the RPA below safe height or block visual line of sight.
• 26/18 — temperature 26°C, dew point 18°C (8°C spread; fog not a concern, but warm moist air suggests a humid air mass).
• Q1018 — QNH 1018 hPa, the altimeter setting.

For a remote pilot the must-check groups are wind (and gust), visibility, cloud base, and temperature/dew point. If any breach the operating limits or the visual-line-of-sight requirement, the flight does not go.

TAF — Terminal Aerodrome Forecast

A TAF (Terminal Aerodrome Forecast) is a forecast of expected weather at an aerodrome over the next 9 to 30 hours (a TAF typically covers 9 or 30 hours depending on the site). It tells you what is expected, with change groups such as BECMG (gradual change), TEMPO (temporary fluctuations), and PROB (probability). A short example:

TAF YSSY 250200Z 2503/2603 18012G20KT 9999 SCT035
      TEMPO 2506/2512 24015G25KT 4000 SHRA

Reading the forecast:

• Initial forecast: wind 180° at 12 kt gusting 20 kt, visibility 10 km, scattered cloud at 3,500 ft.
• TEMPO 2506/2512 — between 06 and 12 UTC, temporary fluctuations are expected: wind veers to 240° at 15 kt gusting 25 kt, visibility drops to 4,000 m in showers of rain (SHRA).

For an RPA operation planned in the TEMPO window, the gust of 25 kt and the 4,000 m visibility in showers are the decisive values; if either breaches the aircraft limit or the operating minimum, the operation should not be planned in that window.

QNH and the Altimeter

QNH is the altimeter pressure setting that causes the altimeter to read aerodrome elevation above mean sea level (AMSL) when the aircraft is on the ground at the aerodrome. It is the pressure value (in hectopascals, hPa) reported at the end of a METAR (e.g., Q1018).

For an RPA the relevance of QNH depends on the aircraft's altitude reference:

• Many small RPA use GPS altitude and do not use QNH directly.
• Aircraft with a barometric altimeter or a barometric altitude reference must have QNH set so the altitude reading corresponds to the local pressure and to the heights published on charts and in airspace bases.
• Even when the aircraft uses GPS altitude, the remote pilot needs QNH to convert between pressure-altitude references on charts (AMSL bases) and the aircraft's altitude display.

BoM Aviation Forecasts and the Go/No-Go Decision

The Bureau of Meteorology (BoM) is Australia's official aviation meteorological service. The products a remote pilot most often uses are:

• METAR and SPECI — observed weather at an aerodrome.
• TAF — aerodrome forecast.
• Graphical Area Forecast (GAF) — a graphical area forecast with charts of cloud, visibility, weather, and QNH, plus a text table.
• Automatic Weather Station (AWS) — observed surface wind, temperature, and pressure at many sites.

The Go/No-Go Decision

A robust go/no-go decision integrates four sources:

1. The latest METAR or AWS observation — wind, gust, visibility, cloud base, and temperature/dew point at a nearby site.
2. The TAF and GAF forecast — what is expected during the operating window, including change groups (BECMG, TEMPO, PROB) and any forecast fronts, thunderstorms, or fog.
3. On-site observation — a hand-held anemometer reading at launch height and a visual scan for building CB, approaching precipitation, fog, or a wind shift the AWS may not yet reflect.
4. The aircraft limits — wind, gust, crosswind, operating temperature, and payload adjusted for density altitude.

The decision rule is conservative: any single source breaching a limit is a no-go. A METAR within limits does not override a TAF TEMPO that forecasts a 25 kt gust during the window; an on-site gust above the aircraft limit is a no-go regardless of what the AWS 5 km away reports.

Key habit: read the METAR, read the TAF, check the GAF, then walk outside and check the wind and sky with your own instruments. The forecast gives you the plan; the observation gives you the decision.