Last week, a German web portal proclaimed that airplanes have a horn that sounds similar to a ship’s whistle. A bitter discussion ensued that in the end was mainly about semantics. I thought I might bring some illumination into the matter by writing a few words about the technical and operational background. First, let’s define what a horn actually is. According to Merriam-Webster, it is “a usually electrical device that makes a noise like that of a horn”, which in turn is “a brass wind instrument [hunting horn, french horn]” or “a wind instrument used in a jazz band [especially trumpet]”. I think we know what those sound like. Wikipedia goes a little further: “A horn is a sound-making device that can be equipped to motor vehicles, buses, bicycles, trains, trams (a.k.a. streetcars in North America), and other types of vehicles. The sound made usually resembles a “honk”. The vehicle operator uses the horn to warn others of the vehicle’s approach or presence, or to call attention to some hazard.”
Let’s go through it one by one. Airplanes actually do have an electric “sound-making device”. The aforementioned discussion referred to the “mechanic call”. In the cockpits of Airbus aircraft, the “MECH PB” (pushbutton) can be found on the “Calls” panel, part of the overhead panel. Located next to buttons with which the pilots can call various stations in the cabin (e.g. purser, flight attendant stations, crew rest compartments), it allows us to call ground staff to the plane’s interphone. While the cockpit and cabin stations have a telephone receiver, a headset is plugged into the service port at the front of the plane by the ground crew, protecting handlers from the loud ambient noise on the ramp and facilitating intelligible conversation. A call system also exists for the reverse direction: A toggle switch on the maintenance panel next to the nose landing gear allows ground staff to call the cockpit. The interphone is used on every flight during pushback (there are backup hand signals in case of technical issues). As the plane is towed out of its parking bay, the ramp agent walks alongside, telling us when it is safe to start the engines and giving commands pertinent to the pushback process. The communication for this process is initiated by the ground crew when they are finished with loading. At this point of the pre-flight process, we have the interphone volume up in the cockpit and ground addresses us by simply speaking into the microphone. So when do we use the mechanic call?
Before some flights, ground handling questions come up that can be answered by interphone without the ramp agent having to take the service stairs up to the cockpit. If these questions arise in the cockpit, we can try to initiate communication with the MECH PB. When it is pushed, a sound and a tiny, easily overlooked light are activated on the outside of the plane. While the light remains on until it is reset, the sound is only audible as long as the button is pushed. I would describe it as a high-pitched, tinny whine. You have to hear it yourself to know how ridiculous it is. It’s nowhere near the horn or honk Merriam-Webster and Wikipedia would like it to be. It really doesn’t do such a big, elegant flying machine justice. In fact, the sound is so feeble that ground staff often don’t hear it and we open the windows, waving to summon them to the headset below. Interestingly enough, the operating manual for the A380 does mention an “external horn” in its system description. So planes do have a horn? Not so fast! I think the authors of the OM wanted to make their lives easy and reduced the horn to a sound-making device, leaving ambiguous its purpose.
In my opinion, it’s the function that makes or breaks a horn. The mechanic call is used exclusively when the airplane is parked. It is not designed to warn others or to call attention to a danger of any sort. As the initial claim was of a horn that sounds like that of a ship, it helps to look at that a bit more closely: “A [ship’s] whistle [or horn] is of utmost help in times of bad weather condition and poor visibility, when alerting the nearby ship becomes imperative. Also an important equipment in times of emergency situations, the whistle is used to beckon audible help from a nearby relief source. […] Moreover, the whistle should be in good working condition with clear audibility. The whistle should be designed to provide prolonged blast at appropriate time intervals.” (Bright Hub Engineering) In heavy fog, airplanes do not rely on acoustic signals to maintain distance, neither in the air nor on the ground. Modern radar systems supervised by air traffic and ground controllers show our precise position. The airplane’s equivalent to the ship’s whistle in an emergency is the ELT (emergency locator beacon). With a much greater range than any sound, it transmits a radio signal to ground stations and satellites, allowing rescue crews to pinpoint our location.
In the end, the question is whether you are a theorist or a realist. The web portal referred to the German word “hupe”, which is the vehicle horn described by Wikipedia. But since the discussion was also in German, many comments quoted German dictionaries and Wikipedia, which define the word simply as a device for creating acoustic signals. Per definition, the mechanic call can be and is named a horn. But for practical purposes, it does not have the same function as and is incomparable to the horn on any vehicle, especially ships. Owing to its acoustic weakness, the mechanic call is not adequate as a means of signaling danger. It is not conspicuous in the loud ramp environment and it can better be compared to the ringer or buzzer of a telephone. Considering the fact that sounds can be activated from both sides and are meant to initiate conversation, that’s actually what it is! Watch the video I made and tell me what you think: do airplanes have a horn?