Switch on transponder at all times
According to SERA.13001, an important principle applies: if a serviceable SSR transponder is on board, it must essentially be operated during the flight – regardless of whether one is currently in airspace where a transponder is mandatory.
An exception exists, inter alia, for aircraft whose electrical supply is insufficient for continuous operation.
The widespread notion that it is better to turn off a transponder outside controlled airspace is therefore not only wrong, but also safety-critical.
From secondary radar to Mode S and ADS-B
Classic primary radar detects an aircraft based on the reflection of transmitted radar signals. A transponder, on the other hand, belongs to the secondary radar system: it responds to interrogations and transmits additional information in the process.
Mode A transmits the four-digit squawk code. Mode C adds the barometric pressure altitude. Mode S also enables an individual 24-bit address and further data.
With a suitably equipped Mode S extended squitter transponder, ADS-B Out is added. In this process, the aircraft regularly transmits data such as its GPS position, barometric and geometric altitude, and its identification.
Other suitably equipped aircraft can receive this information directly and use it for traffic visualisation.
ALT is the normal operating mode in flight
Many transponders have the operating modes OFF, STBY, GND, ON and ALT.
During normal flight operations, the transponder should be set to ALT.
Then he answers queries and additionally transmits the pressure altitude. In the case of a Mode S or ADS-B capable device, the corresponding additional information is transmitted automatically.
The ON position is slightly different: the transponder is operating, but altitude transmission is suppressed.
This operating mode is normally only used when explicitly requested by ATC – for example, when the pressure altitude transmitted by the transponder is clearly incorrect.
GND is intended for ground operation
Many modern Mode S transponders also feature a GND mode.
This is intended for ground operations. The device signals its ground status, among other things, and can continue to respond to corresponding Mode S interrogations. In the case of ADS-B-compatible systems, ADS-B data relevant to ground operations is also transmitted.
This is particularly important at airports with modern surface movement control. Air traffic controllers can clearly identify and track aircraft on taxiways and aprons.
The traffic systems of other aircraft can also benefit from this. Modern avionics can, for example, warn of traffic on or near a runway.
Some devices switch automatically
Not every transponder has a pilot-selectable GND position.
Many modern installations automatically detect whether the aircraft is on the ground or in the air. For this purpose, a weight-on-wheels signal or other avionics information can be used, for example.
The pilot then leaves the system in the designated normal operating mode, while the transponder automatically switches internally between ground and airborne status.
Therefore, the operating manual for the specific installed device is decisive. There is no blanket rule for every transponder type.
Standby is rarely the right choice today
The STBY mode dates back to a time when transponders required a certain warm-up time after being switched on.
In standby, the device works internally, but does not respond to normal queries.
For regular taxiing operations, STBY is therefore usually not provided for in modern systems. Anyone who routinely switches to standby on the ground can prevent the aircraft from being correctly detected by corresponding surveillance systems.
However, for actual operation, the following also applies: the flight manual, avionics manual and, where applicable, local operating procedures are authoritative.
ADS-B-Out and ADS-B-In are two different things
A common misconception concerns the difference between ADS-B Out and ADS-B In.
ADS-B Out means that your own aircraft transmits its data.
By contrast, ADS-B-In means that signals from other aircraft can be received and displayed on a screen.
An aircraft can have ADS-B Out without having ADS-B In – and vice versa.
For collision avoidance, ADS-B Out is particularly important because it allows other appropriately equipped air traffic to electronically detect your own aircraft.
The quality of the GPS position matters
ADS-B does not just transmit a position. The signal additionally contains information about the quality and integrity of this position information.
This includes parameters such as SIL, SDA, NIC and NAC.
A transport system can use these values to assess how reliable the transmitted position is.
This is why not every temporary GPS source is automatically suitable for certified ADS-B Out. In an approved installation, the transponder and position source must be correctly connected to each other and configured accordingly.
Particularly with retrofits, it should therefore be checked whether a system actually delivers standard-compliant ADS-B Out and does not merely transmit position data in some technical way.
ID check upon request only
The IDENT button is frequently misunderstood.
It neither increases transmission power nor suddenly makes the aircraft visible to radar.
When pressed, a special identifier is simply set in the transponder signal for a short time. This allows the corresponding target to be highlighted particularly on the air traffic controller's screen.
According to SERA, IDENT may therefore not be used at will.
The pilot only presses the button when explicitly instructed by ATS, for example with:
„Squawk Ident.“
This allows the controller to clearly assign a specific radar target to the aircraft currently being spoken to.
VFR button: Caution on international flights
Many modern transponders have a VFR button. This allows the locally configured standard squawk to be set with a single press.
In Germany and in many European countries, 7000 is frequently used for uncontrolled VFR flights.
However, this is by no means the same everywhere internationally. In the USA, for example, 1200 is the standard code for VFR.
Before a flight abroad, it should therefore be clarified which code is used in the respective country and which value is programmed in the device behind the VFR button.
The three known emergency codes
Pilots should also know the three internationally important special codes:
- 7500 for unlawful interference or hijacking
- 7,600 for radio failure
- 7700 for a general emergency
With transponders featuring a rotary knob, care should be taken not to inadvertently dial these codes in between when changing codes.
Modern appliances frequently avoid such issues through direct number entry or intelligent operating logic.
ADS-B makes correct operation even more important
In the past, from the perspective of many private pilots, the transponder was primarily an instrument for ATC.
With ADS-B, its significance has changed.
Today, the correct transmission of the transponder can directly contribute to other pilots recognising one's aircraft on their traffic displays.
This does not only apply to the en-route phase. ADS-B-based functions are also gaining importance on the ground. Modern systems can, for example, alert to occupied runways or approaching traffic.
Therefore, a switched-off or incorrectly operated transponder not only reduces the information available to air traffic control, but under certain circumstances also the situational awareness of other airspace users.
Conclusion
The operation of a modern transponder is fundamentally simple, but should be carried out consciously.
During flight, ALT is normally the correct operating mode. On the ground, GND is used on appropriately equipped devices, or the system performs the switch automatically. ON suppresses altitude transmission and is therefore not a normal flight operating mode. IDENT is pressed exclusively upon instruction from air traffic control.
With the increasing prevalence of ADS-B, correct operation is becoming even more important. The transponder is long since no longer just a tool for the air traffic controller. It is part of an increasingly densely networked system for traffic detection and collision avoidance – and thus a directly safety-relevant device in the cockpit.
Source references:
Aviation magazine
