一次搞懂 Minimum Safety Altitude, Decision Height and “Go-around Decision Point”

Reference : Skybrary，Boldmethod

話說今晚又飛機友聚會攞到d 靈感，今晚就學幾個生字啦~

Decision Altitude/Height (DA/DH)

https://skybrary.aero/articles/decision-altitudeheight-dadh

Definition

The Decision Altitude (DA) or Decision Height (DH) is a specified altitude or height in the Precision Approach or approach with vertical guidance at which a Missed Approach must be initiated if the required visual reference to continue the approach has not been established. (ICAO Annex 6)

• Decision altitude (DA) is referenced to mean sea level
• Decision height (DH) is referenced to the threshold elevation (AGL)

• The DH for Category II and III approaches is invariably assessed by reference to a radio altimeter and never a barometric altimeter; therefore the minima can only be expressed as DH and not DA. For approaches with DH of 200ft or higher, radio altimeter reading would be unreliable due to the unevenness of the terrain; therfore a barometric altimeter is always used and the minima may be expressed as DH or DA.

• The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path. In Category III operations, the required visual reference is that specified for the particular procedure and operation.

• For convenience where both expressions are used they may be written in the form “decision altitude/height” and abbreviated “DA/H”

Missed approach must be commenced at the DA/H unless the required visual reference has been established. Calculation of the DA/H takes into account that the aircraft will descend below the DA/H during the missed approach.

For more information regarding the calculation of DA/H see Aerodrome Operating Minima (AOM).

重點：

1. DH/DA 呢個terms 係related to Precision Approach with vertical guidance (eg VNAV?)
2. DH/DA 見吾到required visual reference 就Go around 啦
3. DH 不等於 DA – 睇用CAT幾 （因為測量vertical distance既方法不同）

Minimum Safe Altitude (MSA)

https://skybrary.aero/articles/minimum-safe-altitude-msa#:~:text=Minimum%20Safe%20Altitude%20(MSA)%20is,as%20such%20does%20not%20exist.

Definition

Minimum Safe Altitude (MSA) is a generic expression, used in various cases to denote an altitude below which it is unsafe to fly owing to presence of terrain or obstacles. An ICAO definition of the term “minimum safe altitude” as such does not exist.

Description

Minimum altitudes are calculated in relation to the highest terrain or obstacle within a specified area, allowing a buffer for error, and adding a specified margin.

Minimum altitudes associated with Controlled Airspace may be published in the states’ AIPs.

The expression “minimum safe altitude” is also used in relation to the Minimum Safe Altitude Warning (MSAW) function of ATC systems.

Note. The acronym MSA is also often used as a substitute for “minimum safe altitude”. However, the International Civil Aviation Organisation (ICAO) definition of the acronym MSA is Minimum Sector Altitude (MSA). When the abbreviation MSA is encountered, care should be taken to establish which term it refers to.

重點：

1. ICAO definition 根本無呢個terms, 佢係一個generic expression!
2. 佢係指某區域在考慮highest terrain/obstacle同埋有容許buffer下既最低安全高度
3. 係邊到搵到？如果係controlled airspace 就可以睇AIP

回歸今日topic，到底咩係 “go-around decision point” ? 其實呢個av界有無呢樣野我真係吾知，不過我做看了一些文章再唸唸，呢條問題本身都伏伏地！

Why Calling ‘Go-Around’ Is An Action, Not A Decision Point

https://www.boldmethod.com/learn-to-fly/maneuvers/why-calling-go-around-is-an-action-and-not-a-decsion-point/

Multi-Crew Flight Deck: Seeing Something The Other Pilot Doesn’t

In the airline world, when a crewmember calls a go-around, it’s treated as an action, NOT a decision point. This is for one big reason. Maybe there’s something the “pilot monitoring” sees that you don’t. Every approach should be “flown to a go-around” until a landing assured. It’s one of the reasons that Part 121 operations have such a good safety record.

Go-arounds are penalty-free. On the other hand, ignoring a crewmember’s call for a go-around can lead to serious consequences, especially if there’s an impending threat that caused the pilot monitoring to call “go-around.” In the cases below, two separate pilots ignored go-around calls from their crewmembers. Give them a read, and below we’ll discuss how this applies to general aviation flying.